How many people does each person with pandemic H1N1 infect? This number is often used to estimate how many people have been infected at different times during a pandemic. A study examining the number of people infected within households was recently published in The New England Journal of Medicine by Cauchemez et al. There were several important findings:
An acute respiratory illness developed in 78 of 600 household contacts (13%). In 156 households (72% of the 216 households), an acute respiratory illness developed in none of the household contacts; in 46 households (21%), illness developed in one contact; and in 14 households (6%), illness developed in more than one contact.
Household contacts 18 years of age or younger were twice as susceptible as those 19 to 50 years of age (relative susceptibility, 1.96; Bayesian 95% credible interval, 1.05 to 3.78; P=0.005), and household contacts older than 50 years of age were less susceptible than those who were 19 to 50 years of age (relative susceptibility, 0.17; 95% credible interval, 0.02 to 0.92; P=0.03).
The transmissibility of the 2009 H1N1 influenza virus in households is lower than that seen in past pandemics.
Although the authors don’t comment on this, their results imply that fewer people have been infected with the new H1N1 than many models assume. The natural corollary of this is that pandemic H1N1 is more lethal to those who do become infected than papers that use these models conclude. The other natural conclusion is that there are still many susceptibles, people who have not yet been infected, who could be infected in the future.
Many mainstream media stories reporting on this paper imply that it proves that adults have some natural protection against pandemic H1N1 because they were less likely to be infected than children. This is false. From the paper:
We found that children were twice as susceptible to infection with the 2009 H1N1 virus from a household member as adults 19 to 50 years of age and that adults older than 50 years of age were less susceptible than younger adults. This suggests that the young age distribution that was observed among reported cases in the community (the index patients in our study) was not an artifact resulting from case-ascertainment bias. In addition, our findings are consistent with serologic analyses of the 2009 H1N1 virus suggesting that there are some preexisting pandemic H1N1 immune responses in the elderly; these are present to a lesser extent in younger adults but are rarely present in children. Susceptibility as measured in this analysis captures social and hygienic, as well as biologic, determinants.
In other words, although pre-existing antibodies could explain why adults didn’t sick as often, this study did not examine that issue. Kids could have gotten sick more often because they had poorer hygiene and coughed on each other more than on adults.
This paper confirms other studies suggesting pandemic H1N1 is not yet transmitting efficiently among adults. This is fortunate because such transmission will be associated with large scale absences which may degrade critical infrastructure to dangerous levels. However, the virus is now under intense selection to spread more efficiently, including among adults. Assuming that it will not acquire this ability would be foolish.
Cauchemez et al. (2009) Household Transmission of 2009 Pandemic Influenza A (H1N1) Virus in the United States. New England Journal of Medicine. Volume 361:2619-2627.