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“I say, ‘Use this vaccine,’ ” he said. “The safety profile is actually quite good. But we have oversold it. Use it — but just know it’s not going to work nearly as well as everyone says.”

“Not having evidence doesn’t prove it doesn’t work; we just don’t know,” said Dr. Roger Thomas, a Cochrane Collaboration coordinator for the University of Calgary in Alberta, who was an author of both of the reviews. “The intelligent decision would be to have large, publicly funded independent trials.”

“Does it work as well as the measles vaccine? No, and it’s not likely to. But the vaccine works,” Dr. Joseph Bresee, chief of epidemiology and prevention in the C.D.C.’s influenza division, said. And research is advancing to improve the effectiveness of the vaccine.

The six least-vulnerable countries include all four Scandinavian countries, Germany, and Canada.