Study: H1N1 not highly contagious in households

Maryn McKenna Contributing Writer

Dec 30, 2009 (CIDRAP News) – People living in the same household were less likely to catch H1N1 influenza from a sick family member than they would have been in past pandemics or during a normal flu season, a team from University College London and the Centers for Disease Control and Prevention report today in the New England Journal of Medicine.

Drawing on a database of information gathered about patients with lab-confirmed H1N1 during the pandemic’s early stages, the researchers found the new flu was not very contagious. There were secondary infections in 27% of 216 households and 13% of 600 household residents, compared to secondary attack rates that rose to 20% in the 1957 and 1968 pandemics and up to 40% in some flu seasons. Children and teenagers were twice as susceptible as adults.

The database was assembled from reports filed by state health departments in April and May 2009, while the CDC was still requesting case counts. The reports were written up for any patients whose flu infections were lab-confirmed either as H1N1 flu, or as neither of the H1 and H3 strains that circulated in the 2008-09 flu season.

The reports, filled out during telephone interviews, included information about household size and any other cases in the household. Influenza in the secondary cases was not confirmed by lab tests, however; it was merely described on the basis of symptoms as either “acute respiratory illness” (fever, cough, sore throat and/or runny nose) or “influenza-like illness” (fever and cough or sore throat).

In 72% of the 216 households, no one caught flu from a family member. In 21%, one person did, and in 6%, two or more did. The median age of all 600 household members was 26, but the median age of those who contracted flu from someone was 14.5 to 16.5, depending on how their symptoms were recorded.

The age of the person who brought flu into the household, however, had no effect on their contagiousness; toddlers, teenagers and adults all transmitted flu to others.

The time that it took for flu to pass to a family member—technically, the mean serial interval, between the start of symptoms in the index case and in the first secondary case—was 2.6 days. No one symptom could be linked to H1N1’s being more or less infectious, except that almost all of the patients reported coughing.

For reasons the authors could not explain, the probability of household transmission decreased as the number of household members increased. Among two-person households, 28% of contacts developed flu within 7 days of the first patient’s symptoms starting, but in households with six or more members, only 9% did. “These differences … highlight the fact that the sociologic, environmental, and biologic mechanisms available to explain the relationship between secondary attack rates and household size are still limited,” they said.

The study is significant, the authors said, because so little work has been done on risk factors for flu transmission at home, despite public health recommendations that people who contract flu stay home until 24 hours after their fever subsides. In addition, they said, the findings confirm the observation that children and teenagers are more vulnerable to H1N1 flu than adults.

Cauchemez S, Donnelly CA., Reed C, et al. Household transmission of 2009 pandemic influenza A (H1N1) virus in the United State. N Engl J Med 2009;361(27):2619-27 [Full text]