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The agency cites the results of a study in Germany and clinical trials in the U.K. and Spain as supporting the safety of vaccine interchangeability.

As of late May, 50.6% of Canada's population had received at least one vaccination shot — but only 4.6% of the population was fully vaccinated.

"Different vaccine products have been used to complete a vaccine series for influenza, hepatitis A, and others to complete a vaccine series for influenza, hepatitis A, and others."

"Basically, all vaccines work by showing people's immune systems something that looks like an invading virus but really isn't. If the real virus ever comes along, their immune systems will recognize it and be prepared to fight it off.

"Using two different vaccines is a bit like giving the immune system two pictures of the virus, maybe one face-on and one in profile."

"Individuals who have received one dose of the Pfizer-BioNTech or Moderna COVID-19 vaccine should receive a second dose of the same vaccine to complete the vaccination series," the spokesperson added.

“Please be assured that YOU ARE SAFE after vaccine from what matters — disease and spreading.”

The risks for vaccinated people are still not zero, because almost nothing in the real world is zero risk. A tiny percentage of people may have allergic reactions

the evidence so far suggests that the vaccines are akin to a cure.

The Moderna and Pfizer vaccines — the only two approved in the U.S. — are among the best vaccines ever created, with effectiveness rates of about 95 percent after two doses.

That’s on par with the vaccines for chickenpox and measles.

The Moderna and Pfizer vaccines are “essentially 100 percent effective against serious disease,” Dr. Paul Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said. “It’s ridiculously encouraging.”

“It’s going to save your life — that’s where the emphasis has to be right now,”

“We’re underselling the vaccine,”

Right now, public discussion of the vaccines is full of warnings about their limitations: They’re not 100 percent effective. Even vaccinated people may be able to spread the virus. And people shouldn’t change their behavior once they get their shots.

These warnings have a basis in truth, just as it’s true that masks are imperfect.

"At this point, the Oxford researchers have no idea whether they have something or not," Offit said. "You just get so tired of this 'science by press release.' "

There are currently 10 vaccines in human clinical trials worldwide, according to the World Health Organization. Four of the teams are in the United States: Moderna, Pfizer, Inovio and Novavax.

Moderna CEO Stéphane Bancel referred to the results as "positive interim Phase 1 data" and that "the Moderna team continues to focus on moving as fast as safely possible to start our pivotal Phase 3 study in July."

Moderna is collaborating on its vaccine development with the National Institute of Allergy and Infectious Diseases. Dr. Anthony Fauci, the director of NIAID, said while Moderna's numbers were limited, "it was good news" and he was "cautiously optimistic" about the vaccine.

Inovio and Moderna have said they expect their large-scale clinical trials, known as Phase 3 trials, to last around six months. Pfizer hasn't given a timetable for its Phase 3 trial.

"I've not seen anyone wrap up a Phase 3 trial in a month to six weeks," said Dr. Saad Omer, a Yale University infectious disease expert who's done clinical trials on polio, pertussis and influenza vaccines. "We need to benchmark this against realistic expectations."

"As vaccine researchers like to say, mice lie and monkeys exaggerate," Offit said.

One big stumbling block for any vaccine trial is that Covid-19 infection rates in many areas of the world are flattening out or declining.

The Oxford vaccine uses what's called an adenovirus vector. Adenoviruses cause the common cold, but in this case, the adenoviruses are weakened and modified to deliver genetic material that codes for a protein from the novel coronavirus. The body then produces that protein and, ideally, develops an immune response to it.

"Compared to previous vaccines, this method is more robust, more versatile, and yet, equally efficient," according to the blog, which notes that the Bill & Melinda Gates Foundation invested $53 million in a German biotech company that specializes in RNA vaccines.

Inovio's technology uses a brief electrical pulse to deliver plasmids, or small pieces of genetic information, into human cells. Inovio says those cells then produce the vaccine, which leads to an immune response.

On April 19, the BBC's Andrew Marr said he asked Gilbert "if it's guaranteed that a workable vaccine can actually be produced."

"Nobody can be absolutely sure it's possible. That's why we have to do trials. We have to find out. I think the prospects are very good, but it's clearly not completely certain,"

"It certainly worked in monkeys," Oxford's Hill told CNN's Burnett May 15. "That was quite an impressive impact and that was our first try, if you like, with a standard dose, a single dose of vaccine."

"I buy that this is a pandemic and we may need to show progress and show steps, and I'm OK with making forecasts if decision makers want that, but do it with a level of uncertainty, because that's what's warranted," said Omer, director of the Yale Institute for Global Health.

"Now researchers can't wait to step out to the microphone -- and there are so many microphones out there -- to say, 'I've got it! This looks really good!' " Offit said.

Alaska became the first state to make vaccinations available to all adults over the age of 16 earlier this month, followed by Mississippi. Several others have since followed suit, including Arizona, Utah, Indiana, Georgia and West Virginia.

Other states are moving to make more groups eligible ahead of schedule, based on age or underlying conditions.

According to a map released by the White House COVID-19 Response Team on Friday, four states have yet to confirm plans to expand eligibility ahead of the May 1 deadline: New York, Wyoming, Arkansas and South Carolina, where officials have said they are not on track to hit that threshold until May 3.

Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said at the briefing that the country has seen an uptick in case counts and hospital admissions, with the most recent 7-day averages showing about 57,000 cases and 4,700 hospitalizations per day, and hospitalizations hovering around 1,000.

The U.S. is administering 2.5 million shots a day at its current pace, Zients said, adding that vaccine makers are "setting and hitting targets." Some 27 million doses went to states, tribes and territories this week.

Johnson & Johnson has accelerated production of its single-shot vaccine and is on track to deliver 11 million doses next week.

“My first and biggest wish is that we had known early that Covid-19 was airborne,”

, “Once you’ve realized that, it informs an entirely different strategy for protection.” Masking, ventilation and air cleaning become key, as well as avoiding high-risk encounters with strangers, he says.

Instead of washing our produce and wearing hand-sewn cloth masks, we could have made sure to avoid superspreader events and worn more-effective N95 masks or their equivalent. “We could have made more of an effort to develop and distribute N95s to everyone,” says Dr. Volckens. “We could have had an Operation Warp Speed for masks.”

We didn’t realize how important clear, straight talk would be to maintaining public trust. If we had, we could have explained the biological nature of a virus and warned that Covid-19 would change in unpredictable ways.  

We didn’t know how difficult it would be to get the basic data needed to make good public-health and medical decisions. If we’d had the data, we could have more effectively allocated scarce resources

In the face of a pandemic, he says, the public needs an early basic and blunt lesson in virology

and mutates, and since we’ve never seen this particular virus before, we will need to take unprecedented actions and we will make mistakes, he says.

Since the public wasn’t prepared, “people weren’t able to pivot when the knowledge changed,”

By the time the vaccines became available, public trust had been eroded by myriad contradictory messages—about the usefulness of masks, the ways in which the virus could be spread, and whether the virus would have an end date.

, the absence of a single, trusted source of clear information meant that many people gave up on trying to stay current or dismissed the different points of advice as partisan and untrustworthy.

“The science is really important, but if you don’t get the trust and communication right, it can only take you so far,”

people didn’t know whether it was OK to visit elderly relatives or go to a dinner party.

Doctors didn’t know what medicines worked. Governors and mayors didn’t have the information they needed to know whether to require masks. School officials lacked the information needed to know whether it was safe to open schools.

Had we known that even a mild case of Covid-19 could result in long Covid and other serious chronic health problems, we might have calculated our own personal risk differently and taken more care.

just months before the outbreak of the pandemic, the Council of State and Territorial Epidemiologists released a white paper detailing the urgent need to modernize the nation’s public-health system still reliant on manual data collection methods—paper records, phone calls, spreadsheets and faxes.

While the U.K. and Israel were collecting and disseminating Covid case data promptly, in the U.S. the CDC couldn’t. It didn’t have a centralized health-data collection system like those countries did, but rather relied on voluntary reporting by underfunded state and local public-health systems and hospitals.

doctors and scientists say they had to depend on information from Israel, the U.K. and South Africa to understand the nature of new variants and the effectiveness of treatments and vaccines. They relied heavily on private data collection efforts such as a dashboard at Johns Hopkins University’s Coronavirus Resource Center that tallied cases, deaths and vaccine rates globally.

For much of the pandemic, doctors, epidemiologists, and state and local governments had no way to find out in real time how many people were contracting Covid-19, getting hospitalized and dying

To solve the data problem, Dr. Ranney says, we need to build a public-health system that can collect and disseminate data and acts like an electrical grid. The power company sees a storm coming and lines up repair crews.

If we’d known how damaging lockdowns would be to mental health, physical health and the economy, we could have taken a more strategic approach to closing businesses and keeping people at home.

t many doctors say they were crucial at the start of the pandemic to give doctors and hospitals a chance to figure out how to accommodate and treat the avalanche of very sick patients.

The measures reduced deaths, according to many studies—but at a steep cost.

The lockdowns didn’t have to be so harmful, some scientists say. They could have been more carefully tailored to protect the most vulnerable, such as those in nursing homes and retirement communities, and to minimize widespread disruption.

Lockdowns could, during Covid-19 surges, close places such as bars and restaurants where the virus is most likely to spread, while allowing other businesses to stay open with safety precautions like masking and ventilation in place.  

The key isn’t to have the lockdowns last a long time, but that they are deployed earlier,

If England’s March 23, 2020, lockdown had begun one week earlier, the measure would have nearly halved the estimated 48,600 deaths in the first wave of England’s pandemic

If the lockdown had begun a week later, deaths in the same period would have more than doubled

It is possible to avoid lockdowns altogether. Taiwan, South Korea and Hong Kong—all countries experienced at handling disease outbreaks such as SARS in 2003 and MERS—avoided lockdowns by widespread masking, tracking the spread of the virus through testing and contact tracing and quarantining infected individuals.

With good data, Dr. Ranney says, she could have better managed staffing and taken steps to alleviate the strain on doctors and nurses by arranging child care for them.

Early in the pandemic, public-health officials were clear: The people at increased risk for severe Covid-19 illness were older, immunocompromised, had chronic kidney disease, Type 2 diabetes or serious heart conditions

t had the unfortunate effect of giving a false sense of security to people who weren’t in those high-risk categories. Once case rates dropped, vaccines became available and fear of the virus wore off, many people let their guard down, ditching masks, spending time in crowded indoor places.

it has become clear that even people with mild cases of Covid-19 can develop long-term serious and debilitating diseases. Long Covid, whose symptoms include months of persistent fatigue, shortness of breath, muscle aches and brain fog, hasn’t been the virus’s only nasty surprise

In February 2022, a study found that, for at least a year, people who had Covid-19 had a substantially increased risk of heart disease—even people who were younger and had not been hospitalized

respiratory conditions.

Some scientists now suspect that Covid-19 might be capable of affecting nearly every organ system in the body. It may play a role in the activation of dormant viruses and latent autoimmune conditions people didn’t know they had

 A blood test, he says, would tell people if they are at higher risk of long Covid and whether they should have antivirals on hand to take right away should they contract Covid-19.

If the risks of long Covid had been known, would people have reacted differently, especially given the confusion over masks and lockdowns and variants? Perhaps. At the least, many people might not have assumed they were out of the woods just because they didn’t have any of the risk factors.

Regulatory review and approval. If a medication or drug is proven safe and effective in clinical trials, the U.S. Food and Drug Administration gets involved to evaluate the vaccine and administer an approval. The International Federation of Pharmaceutical Manufacturers and Associations reports that, typically, 1 in 10 experimental vaccines make it all the way through regulatory approval.

Each one of those steps can take years, and a potential vaccine can get stalled indefinitely at any one of them.

There's not a whole lot that can be done to speed up the process and still arrive at a safe and effective vaccine. Currently, most medical and public health experts say we're at least 12 to 18 months away from having a usable vaccine against COVID-19.

Because of the lengthy timeline associated with vaccine development, nearly three dozen companies and academic institutions around the world are now directing resources towards the search

While these new approaches could speed a vaccine to market, it does raise some ethical questions about safety. It's also unclear just yet whether the rush will result in an effective vaccine faster

"In the beginning of the process, the research usually involves searching through tons of sources of data to uncover opportunities that may not be so obvious.

The trial will assess "safety and antibody production, meaning that testing various doses' safety and whether these doses are producing an immune response. This phase 1 trial is not studying the effectiveness of the vaccine in preventing coronavirus infection. That will come at a later phase of the research,

But the sense of urgency surrounding the need for a safe, effective vaccine to prevent COVID-19 is driving public health officials, private pharmaceutical companies and others to work as quickly as they can to find a solution. The sooner these vaccines and other medications can be tested, the sooner we might have a viable vaccine that can halt a global pandemic that shows few signs of slowing on its own. But right now, experts say that it will take at least a year and likely longer before such a vaccine is available

In states across the country, demand for the vaccine has far outweighed supply

Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems.

Other states have decided to remain focused exclusively on vaccinating health care workers and nursing home residents before opening up to a wider swath of the public.

Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months.

At least 28 states and Washington, D.C., have begun vaccinating older people, a New York Times survey shows, in many cases marking a shift in earlier plans that had put medical workers and nursing home residents at the front of the line for Covid-19 inoculations.

“I couldn’t hug her,” Ms. Johnson said. “We didn’t share food. We each brought our own sandwich. It was just surreal.”

The guilt I felt about this compelled me to objectively review the literature on mRNA vaccines. Not being an expert in virology or biochemistry, I realized I had to quickly master unfamiliar words like “transfection” and concepts about gene sequences. Slowly, the information I was devouring started changing my beliefs.

I learned that research into using mRNA for vaccinations and cancer therapies has been ongoing for the past 30 years. Trial and error have refined this modality so that it was almost fully fledged by the time Covid hit

The mRNA from the vaccine is broken down quickly in our cells, and the coronavirus spike protein is expressed only transiently on the cell surface.

Furthermore, this type of vaccine is harnessing a technique that viruses already use.

It was humbling to have to change my mind. As I booked my vaccination time slot, I realized how lucky I am to have access to all this research, as well as the training to understand it.

As medical professionals, we cannot afford to be paternalistic and trust that people will follow advice without all the facts. This is especially true in Australia, where the vast majority of us have never witnessed firsthand the ravages that this disease can inflict.

Like all new converts, I am now a true believer: I’d like everyone to be vaccinated. But autonomy is a precious tenet of a free society, and I’m glad the ethicists have advised against mandating the vaccine

just hope that with more robust discussion and the wider dissemination of scientific knowledge, we may sway people like me — who have what may be valid reservations — to get the vaccine.

He said only those who have had the disease understand how he feels. He had to get his hand on a vaccine quickly. But most of Tijuana's limited supply was going to front-line hospital workers.

He insists that no one at the vaccination site checked whether he worked or resided in the county, the two requirements necessary to get a shot there.

But vaccine tourism has become a bit of a phenomenon in Mexico. It's easy to find testimonies and tips on social media and in chat groups about getting a vaccine in the U.S.

The Biden administration has said the United States will send Mexico more than 2 million COVID-19 vaccine doses.

Earlier this month, President Biden said, "We're going to start off making sure Americans are taken care of first and then we're going to help the rest of the world."

"In this particular case, amid a worldwide pandemic, life and health of everyone should be priority No. 1,"

patients will not be able to choose which of the two vaccines they get.

the one produced by Bharat Biotech is being deployed prematurely. It still has yet to clear phase three clinical trials, and efficacy data isn't expected until March

Bharat Biotech's founder & chairman, Krishna Ella, told a Jan. 4 news conference his company's vaccine is "200% safe."

But there's a difference between giving an experimental drug to someone who is already sick, and giving a vaccine to someone who is healthy, says public health activist Dinesh Thakur.

Making vaccines — or least, mass-producing them — is something India is actually famous for. It's the world's largest vaccine producer, nicknamed 'the pharmacy to the world.'

with 42 states reporting downward trends

Moderna says its vaccine protects against some variants

The good news, Fauci told CNN in a separate interview Monday, is that current Covid-19 vaccines are likely to be effective against the new variants.

Moderna said Monday its vaccine created antibodies that neutralized Covid-19 variants first found in the UK and South Africa

So far, about 19 million people -- nearly 6% of the US population -- have received at least the first dose of the Covid-19 vaccine, according to CDC data. More than 3.3 million are fully vaccinated.

100 million shots administered in the President's first 100 days in office.

Kentucky has used about 88% of their first doses

The country will also need to open more vaccination sites, some of which should be open 24/7 to ensure people who get off work at odd hours have easy access to the vaccine.“This is a wartime sort of response,” Fagbuyi said. “You’re going to pull all the stops and not leave any stone unturned.”

“Enjoy the opportunity, but also be cautious and look at what’s going on around you,” Fagbuyi said.

“We’re going to be trying to convince people to come get vaccinated,” Adalja said.

Adalja predicts that by the Fourth of July, vulnerable people will be vaccinated in all states and there will no longer be concerns about reaching hospital capacity.

So, when will we get to a point where it won’t be so complicated for the general public to score a vaccine appointment? Fagbuyi predicts sometime in June.

We often hear doctors say we want 70% of the population to be immune to COVID-19, whether through vaccination, natural infection or preexisting immunity, to achieve herd immunity. Each dose pricked in someone’s arm makes it harder for the virus to spread.

“What I think is more important than herd immunity is making sure vulnerable populations are vaccinated so that COVID loses the ability to cause severe illness, hospitalization and death,” Adalja said.

She and others emphasized the apparent lack of severe health repercussions from reinfection — and the lack of evidence that reinfection is common.

Nearly 4 percent of people who had a previous infection were reinfected, an almost identical rate to those with no history of infection.

“Basically, it’s saying vaccination actually needs to be better than natural immunity. But vaccination is better than natural immunity.”

The study backs up recent laboratory data from South African researchers analyzing blood plasma from recovered patients. Nearly half of the plasma samples had no detectable ability to block the variant from infecting cells in a laboratory dish

The good news is that vaccine trials from Johnson & Johnson and Novavax show that vaccines can work — even against the B.1.351 variant, and particularly in preventing severe illness.

Novavax did not provide a breakdown of mild, moderate and severe cases, but severe cases of covid-19 were rare in the trial, suggesting that reinfection is unlikely to send people to the hospital.

“It is not surprising to see reinfection in individuals who are convalescent. And it would not be surprising to see infection in people who are vaccinated, especially a few months out from vaccine,”

“The key is not whether people get reinfected, it’s whether they get sick enough to be hospitalized.

“If the data holds true, it means we will need to walk the public back on the idea of how close we are to the finish line for ending this pandemic.”

Projections created by data scientist Youyang Gu — whose pandemic models have been cited by the Centers for Disease Control and Prevention — suggest that about 65 percent of America’s population will reach immunity by June 1. But built into that 65 percent is roughly 20 percent having immunity from past infections only.

In a separate study, scientists at Rockefeller University in New York took blood plasma from people who had been vaccinated and found that vaccine-generated antibodies were largely able to block mutations found on the B.1.351 variant.

I think the fact that we … now have data from two vaccines indicating that we can prevent serious disease, even against the new variant, is hopeful,”

A future concern needing close monitoring is whether the reformulation of vaccines to keep up with the evolving virus could drive the virus to continue evolving.

There is also a concern that subpar immunity could allow new resistant variants to emerge. That possibility, Nussenzweig said, is one reason that people should get both doses of a vaccine, on time.

“Usually by four to six weeks, there’s not much left,” said Deepta Bhattacharya, an immunologist at the University of Arizona. But germinal centers stimulated by the mRNA vaccines are “still going, months into it, and not a lot of decline in most people.”

The broader the range and the longer these cells have to practice, the more likely they are to be able to thwart variants of the virus that may emerge.

“Everyone always focuses on the virus evolving — this is showing that the B cells are doing the same thing,” said Marion Pepper, an immunologist at the University of Washington in Seattle. “And it’s going to be protective against ongoing evolution of the virus, which is really encouraging.”

Dr. Ellebedy’s team found that 15 weeks after the first dose of vaccine, the germinal center was still highly active in all 14 of the participants, and that the number of memory cells that recognized the coronavirus had not declined.

“The fact that the reactions continued for almost four months after vaccination — that’s a very, very good sign,” Dr. Ellebedy said. Germinal centers typically peak one to two weeks after immunization, and then wane.

After an infection or a vaccination, a specialized structure called the germinal center forms in lymph nodes. This structure is an elite school of sorts for B cells — a boot camp where they become increasingly sophisticated and learn to recognize a diverse set of viral genetic sequences.

The results suggest that a vast majority of vaccinated people will be protected over the long term — at least, against the existing coronavirus variants

But older adults, people with weak immune systems and those who take drugs that suppress immunity may need boosters; people who survived Covid-19 and were later immunized may never need them at all.

In the absence of variants that sidestep immunity, in theory immunity could last a lifetime, experts said. But the virus is clearly evolving.

“Anything that would actually require a booster would be variant-based, not based on waning of immunity,” Dr. Bhattacharya said. “I just don’t see that happening.”

The good news: A booster vaccine will probably have the same effect as prior infection in immunized people, Dr. Ellebedy said. “If you give them another chance to engage, they will have a massive response,” he said, referring to memory B cells.

Dr. Ellebedy said the results also suggested that these signs of persistent immune reaction might be caused by mRNA vaccines alone, as opposed to those made by more traditional means, like Johnson & Johnson’s

But that is an unfair comparison, because the Johnson & Johnson vaccine is given as a single dose, Dr. Iwasaki said: “If the J & J had a booster, maybe it will induce this same kind of response.”

The trial’s first participants, a pair of 9-year-old twin girls, were immunized at Duke University in North Carolina on Wednesday.

Immunizing children will help schools to reopen as well as help to end the pandemic, said Dr. Emily Erbelding, an infectious diseases physician at the National Institutes of Health who oversees testing of Covid-19 vaccines in special populations.

An estimated 80 percent of the population may need to be vaccinated for the United States to reach herd immunity, the threshold at which the coronavirus runs out of people to infect.

“we were encouraged by the data from the 12 to 15 group,

Scientists will test three doses of the Pfizer vaccine — 10, 20 and 30 micrograms — in 144 children. Each dose will be assessed first in children 5 through 11 years of age, then in children ages 2 through 4 years, and finally in the youngest group, six months to 2 years.

After determining the most effective dose, the company will test the vaccine in 4,500 children

“It sounds like a good plan, and it’s exciting that another Covid-19 vaccine is moving forward with trials in children,”

Children represent 13 percent of all reported cases in the United States.

More than 3.3 million children have tested positive for the virus, at least 13,000 have been hospitalized and at least 260 have died

“We don’t know what the long-term effects of Covid infection are going to be,” Dr. Maldonado said.

Other vaccines have helped to control many horrific childhood diseases that can cause long-term complications, she added: “For some of us who’ve seen that, we don’t want to go back to those days.”

“So there’s a higher degree of confidence now in giving this vaccine to kids.”

“The more transparent you can be, the better.”

those at highest risk for getting sick from Covid-19, either because of their job or their underlying health status, should get first dibs.

These guidelines are supported by bioethical principles about the need to balance "priorities of minimizing societal disruption and preventing morbidity and mortality."

As a result, the system is open to manipulation

A first distribution option is to treat vaccination like a class list.

seniors lined up overnight to get the shot, putting their own health at risk for the chance to avert future infections.

This strategy clearly prioritizes those who are in-the-know, who have time to sit on hold, or who are willing (in the case of camping out overnight) to put their health at risk.

it's the conditions of scarcity that make fairness difficult to achieve.

Whether it's using the National Guard to assist in setting up vaccination centers, deploying mobile vans to access rural and urban populations, or working with community groups to increase uptake among at-risk populations, better is needed.

we must release all the available doses of the Covid-19 vaccines

To protect our communities, we must not be paralyzed by fairness -- or stop pursuing it.

how can the public trust the right decisions were made

how can the public trust the right decisions were made

How can knowledgeable independent healthcare professionals endorse those decisions when they are not privy to the process?

Pfizer vaccine to have a 90% short term efficacy, and a single dose of the Oxford vaccine to have a 70% short term efficacy.

Would confidence in the vaccine be reduced by multiple reports of vaccine failures among at-risk individuals

Perhaps the UK could consider instead other ways to conserve vaccine supplies that may be safer than the delayed second dose option.

Also, for people who have recovered from Covid-19 a single vaccine dose may indeed be sufficient to boost their antibodies to high levels

They can help with decisions about who gets the vaccine first when supplies are limited.

"One of those is how much is the virus spreading as the vaccine is being rolled out? And another factor is. How fast is the vaccine being rolled out?"

It's also important to know how effective a vaccine is at preventing disease, how long protection lasts, and whether it not only prevents someone from getting sick but also from transmitting COVID-19.

Larremore says to end a pandemic, it generally makes sense to vaccinate those most capable of spreading disease.

But even if a mathematical model suggests the most effective path, it doesn't provide all the answers public health officials need.

Right now, modelers are trying to help public health officials decide if it makes sense to use a single dose of the Moderna and Pfizer vaccine to extend the limited supply, even though the vaccine has only really been tested using a two-dose regimen

“We have not seen any variants evade the vaccination completely,” Javaid said.

Right now we use antibody testing as a marker of an immune response. But we need more time to pass to study the population’s response to the vaccines before being able to sufficiently assess the duration of immunity.

Oxford-AstraZeneca

Pfizer-BioNTech

Chinese Vaccines

Moderna

Scientists have developed dozens of Covid-19 vaccines at record speed.

Efficacy: Unknown

pledged to be the primary vaccine provider for the developing world.

claims that its vaccine has a 79 percent efficacy rate, though it has not provided data.

expected to release results from its late-stage clinical trial this month

is estimated to have an efficacy rate between 63 and 78 percent

made waves in August when early data showed that its Covid vaccine prompted a surprisingly robust immune response in people and monkeys.

first Covid-19 vaccine to get emergency authorization in the United States

offers terrific protection against Covid-19 and sometimes comes with mild side effects.

in Britain, India and several other countries

Sputnik V vaccine has an efficacy rate of 91.4 percent.

could have a big impact on the pace of vaccinations in the U.S. because it is given in one dose instead of two.

cheap and easily stored

though data from late-stage trials has not yet been shared publicly.

Many scientists were puzzled, however, by data showing that its efficacy may depend on the strength of the initial dose or the gap between doses.

Moderna’s does not need to be stored at ultracold temperatures, making it better suited for smaller clinics and remote areas.

No serious health problems have been linked to the shot, though some people get fatigue, fever and muscle aches.

run into big delays

being distributed in Argentina, Belarus and other countries.

final stage of testing in the U.S. in late December.