# 4203
A study today from the NEJM which suggests the novel H1N1 virus isn’t as easily transmissible as other novel pandemic viruses of the past.
In a study of 216 households where an index case of H1N1 infection was identified, only about 13% of household contacts (about 1 in 8) came down with the infection.
Looked at another way, in roughly 72% of the households where a case was identified, no one else came down with the virus. In 21% of the households, only one additional member acquired the infection. And in only 6% of households did more than one other family member fall ill.
Children and teens under the age of 18 were twice as likely to catch the virus than those aged 19 to 50. Those over the age of 50 were even less likely to fall ill.
For reasons that are less than clear, the rate of viral transmission in households with just two members was much higher than in households with four or more members.
Excerpts from the abstract are below, but follow the link to read the entire study.
Household Transmission of 2009 Pandemic Influenza A (H1N1) Virus in the United States
Simon Cauchemez, Ph.D., Christl A. Donnelly, Sc.D., Carrie Reed, D.Sc., Azra C. Ghani, Ph.D., Christophe Fraser, Ph.D., Charlotte K. Kent, Ph.D., Lyn Finelli, Dr.P.H., and Neil M. Ferguson, D.Phil.
ABSTRACT
Background As of June 11, 2009, a total of 17,855 probable or confirmed cases of 2009 pandemic influenza A (H1N1) had been reported in the United States. Risk factors for transmission remain largely uncharacterized. We characterize the risk factors and describe the transmission of the virus within households.
Methods Probable and confirmed cases of infection with the 2009 H1N1 virus in the United States were reported to the Centers for Disease Control and Prevention with the use of a standardized case form. We investigated transmission of infection in 216 households — including 216 index patients and their 600 household contacts — in which the index patient was the first case patient and complete information on symptoms and age was available for all household members.
Results 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.
The proportion of household contacts in whom acute respiratory illness developed decreased with the size of the household, from 28% in two-member households to 9% in six-member households. 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).
Infectivity did not vary with age. The mean time between the onset of symptoms in a case patient and the onset of symptoms in the household contacts infected by that patient was 2.6 days (95% credible interval, 2.2 to 3.5).
Conclusions The transmissibility of the 2009 H1N1 influenza virus in households is lower than that seen in past pandemics. Most transmissions occur soon before or after the onset of symptoms in a case patient.
A few caveats to this study are in order.
First, this study took place very early in the outbreak, essentially during the first 3 weeks of May.
While it may not have appreciably changed, this is a snapshot of the transmissibility of the virus at that time.
Second, the data was collected by interviewing (telephone or face-to-face) family members within 7 days of an index case having been identified in their household.
Illnesses that fell outside of that 7 day follow up period were not directly identified.
Third, reports of ILI (influenza-like illness) among family members or contacts within 7 days were assumed to have been due to exposure to the index case.
Testing for H1N1 wasn’t always done, and the actual chain of infection may have been other than from the index case.
None of which is mentioned to negate the findings of this study, but simply stated to illustrate that these sorts of studies have limitations, and those must be considered when you view the data.
This study does seem to suggest that compared to teenagers and children, those over the age of 18 may have some limited immunity to the virus. And that immunity appears to increase with age, becoming more prevalent in those over the age of 50.
The authors put it this way :
. . . 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.
Just as I was about to post this entry, I noticed that Maryn McKenna, writing for CIDRAP, has posted her summary of the report. You’ll find it at the link below.
