People in households where other people had been infected with H1N1 had triple the risk of infection compared with the general population. On the other hand, older people were less likely to be infected, as were people who had existing antibodies.
Why would some people have existing antibodies to an entirely new viral strain? Several reasons, said Chen, who is an associate consultant at Singapore's Tan Tock Seng Hospital. "Some individuals vaccinated [with the seasonal influenza vaccine] developed antibodies to H1N1-2009," he said. "Another is the imperfect specificity of the test for influenza antibodies."
Individuals may also have been infected with older strains of influenza (notably one circulating in 1957-1958), which could have conferred protection against H1N1.
Hospital workers may have been protected through various infection-control measures, such as wearing masks, and patient and visitor screening before entering facilities, the authors stated.
The study did not include information on the spread of the virus in children, which could represent a major drawback.
"We believe that the key players in the outbreak were children," Bromberg said.
And there's some question as to whether the results are applicable to the United States, although Chen believes they are.
"I believe the general finding that large numbers of adults remain susceptible should be broadly applicable," he said. "However, several differences in the population studied should be noted. For instance, the U.S. had two epidemic waves (spring-summer and autumn-winter), while we had one."
"The question is what's going to happen next winter," Blumberg said. "Is the virus going to be worse, the same, is it going to be better? We don't
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