Seasonal Flu Vaccine and Protection Against Pandemic Flu

Brief answer: None.

Longer answer: Recent surveillance data from 356 adults in 8 states indicate that the likelihood of contracting pandemic flu is unaffected by receiving a seasonal flu vaccine. Results of the data analysis are available in the latest issue of MMWR.

After adjusting for age and chronic medical conditions,* the CDC found that the overall effectiveness of the seasonal flu vaccines against contracting pandemic flu was -10%, with a wide 95% confidence interval (CI) straddling zero (-43%, +15%). Data were obtained during the period from May to June of this year.

These epidemiologic data complement known serologic (laboratory) data, which suggest that the trivalent seasonal flu vaccines are unlikely to provide meaningful cross-protection against pandemic flu. Likewise a recent Australian study showed that, in children or adults who received seasonal flu vaccines, cross-reactive antibodies against pandemic flu were not produced. The overall age-adjusted effectiveness of the seasonal flu vaccines against pandemic flu in this study was 3% (CI: -56%, +40%). An unpublished study of high schoolers in New York City also suggests no protective benefit of seasonal flu vaccines against pandemic flu.

However, these data are at odds with those from a recently published Mexican study, which reported a seasonal flu vaccine effectiveness rate of 73% against pandemic flu. But control patients in this study were more likely to have chronic medical conditions—making seasonal flu vaccine coverage more likely in this group. Unpublished data from Canada suggest that "medically attended" pandemic flu may actually be more likely in recipients of the seasonal flu vaccine.

The MMWR editors advise that studies with more rigorous designs and methods are currently under way to evaluate any immunity conferred by seasonal flu vaccines against pandemic flu.

* Which increase the risk of influenza-related complications (eg, heart disease, asthma, diabetes, cancer).