Tracking the Household Transmission of H1N1 (Swine Flu) Influenza Virus

The 2009 flu season has been met with increased public concern due to the emergence of H1N1 (swine flu) influenza virus. Concern is over not only the virus itself, but also the vaccine used to prevent its spread. The H1N1 swine flu virus contains genetic information from the influenza virus that infects swine, enough so to necessitate the production of a novel vaccine to protect the public.

A new study in the New England Journal of Medicine assessed aspects of household spread of the virus in the United States. The goal of the work was to determine what factors may promote the spread of swine flu within a household and who may be more susceptible, allowing for greater public awareness.

The investigators followed 216 patients with confirmed, or probable, H1N1 flu and their 600 household contacts. A contact was defined as someone who stayed at the patients' residence for at least one night a week before or after the onset of flu symptoms. Patients must have had a fever of 100 degrees Fahrenheit, a cough or sore throat, and tested positive for the H1N1 swine flu virus using polymerase chain reaction diagnostics. Patients could also be what the authors deemed "probable", which is stated as: positive for influenza A and negative for H1 or H3 human flu virus.

Contacts did not have to be confirmed to have the H1N1 virus, they were assumed to be infected if they developed "acute respiratory illness", which the authors defined as the presence of two of the following: fever, cough, sore throat, or runny nose. Contacts were required to acquire symptoms within 7 days of the original patient they resided with to be considered.

The authors found that 13% of contacts, based upon their criteria, contracted "acute respiratory illness." The infected contacts were stratified by age to assess whether certain age groups were more susceptible, as other reports suggest that older individuals may have some cross reactivity to the virus due to previous infections or immunizations.

Similar to previous data, younger individuals were at higher risk for assumed H1N1 infection. Individuals between the ages of 0-4 had a relative risk of 2.33, and ages 5-18 a relative risk of 1.6. The relative risks for the 19-50 and 50 or above age groups were not statistically significant (close to 1). This suggests that someone in the age group of 0-4 is 2.33 times more likely to contract swine flu than those in the older age groups.

It is unclear if the data generated has a molecular basis or is due to another factor, such as reduced hygiene (i.e. less hand washing, putting fingers in mouth). Nevertheless, the data the authors presented suggests that parents should be mindful of who their young children come in contact with during this flu season, as they appear to have a higher susceptibility for contracting swine flu.

References:

Cauchemez, S. et, al. New England Journal of Medicine. 2009; 361:2619-27