Since 2008, the Center for Disease Control and Prevention (CDC) expanded recommendations for the target population of flu vaccination in the United States. Previously, routine flu vaccination was not recommended for healthy non-pregnant adults 18-49 years of age who were without occupational or contact risks for influenza. During the emergence of the H1N1 pandemic flu in 2009-2010, a significant number of people in this “low risk” group were noted to be severely affected by influenza related complications. For this 2010-2011 flu season, the CDC has again expanded the vaccination recommendations to now include all persons 6 months of age and older who do not have a contraindication to the vaccine. Also new will be the inclusion of the 2009 H1N1 pandemic flu strain in this year’s seasonal flu vaccine. With expanded vaccination recommendations, and a relatively new H1N1 influenza vaccine, patients are pouring in with questions and concerns about the effectiveness and side effects of the H1N1 influenza vaccine.
In this week’s NEJM, a team lead by Drs. Jiang Wu and Fujie Xu, describe the efficacy and safety findings following a 5 day mass vaccination program in Beijing. In the program 95,244 children and adults received the monovalent 2009 pandemic H1N1 influenza vaccine. Lindsey Baden, NEJM deputy editor and infectious disease specialist, commented, “Until now, much of the data regarding the 2009 H1N1 pandemic influenza vaccine has described immunogenicity. This article provides a large sample population to evaluate the actual clinical effectiveness of the vaccine, and its safety profile.”
Vaccine effectiveness was evaluated by comparing 25,037 vaccinated students to 244,091 non-vaccinated students from the same schools. A total of 9 confirmed cases of 2009 H1N1 virus infection were reported in the vaccinated cohort, and 687 cases in the unvaccinated cohort – translating into a vaccine effectiveness of 87.3 %.
Adverse events were evaluated through a designated diary card surveillance system. The overall proportion of vaccine recipients who reported any adverse event was 193 out of 95,244 people (0.2%), with the most common adverse event being fever. On further review, only 81 of the 193 reported adverse events were deemed to be vaccine related. All the side effects resolved without the need for hospitalization. To assess for the often particular concern of neurological symptoms, such as Guillain-Barre syndrome, active surveillance for neurologic diseases was instituted in 31 hospitals in the Beijing metropolitan area. During the surveillance period, a total of 362 patients had an incident neurologic condition, including 27 cases of Guillain-Barre syndrome and 105 cases of optic neuritis. None of these cases occurred in patients who were vaccine recipients.
As the flu season is underway in many countries this information from Beijing’s 2009-2010 flu season and H1N1 flu vaccination campaign is helpful. Of course, questions arise. Will these trends persist in different countries? Will patients be reassured by the safety profile of the flu vaccination in Beijing? Ongoing surveillance and time will tell if looking back has affected the future patterns of H1N1 influenza and vaccination.