2012-13 Seasonal Influenza–an update!
By now most people are aware that this season’s influenza outbreak is more serious in comparison to the past several years. Case numbers have been increasing rapidly, and have done so earlier in the season, which has led to a couple of reports that I’ve noticed calling the effectiveness of the vaccination campaign into question. Although the number of deaths attributable to influenza is below the epidemic baseline level determined by the Centers for Disease Control and Prevention (meaning: the percent of deaths attributable to influenza has not risen above a level above the normal level of influenza deaths predicted for the same time period), the number of cases is significantly higher. In Pennsylvania alone, the number of cases has been high enough to tax hospital resources, prompting the Lehigh Valley Hospital at Cedar Crest to set up an influenza tent in the parking lot.
The severity of the outbreak despite easy availability of the vaccine has prompted me to summarize what is known of the varieties of flu circulating this season. This is important, as many people may question “Why get a flu shot? It isn’t working anyway” which may potentially worsen an already serious situation. Estimates by the CDC currently find that approximately one third of the general US population has already received the seasonal flu vaccine, as has approximately one half of health care professionals. These numbers typically rise about 10% more by the end of the flu season, both in the general population and among health care workers.
Seasonal influenza vaccines (either the attenuated or “live” vaccine, and the inactivated or “killed” vaccine) are known as component vaccines, containing the antigens from several varieties of influenza virus. The choice of components is made by the World Health Organization, after surveying the most significant virus varieties early in the season. The effectiveness of the vaccine therefore is partially determined by how well this collaborative group “predicts” what influenza varieties are going to be circulating. This year’s influenza component vaccine contains three varieties of virus: a variety of H1N1 influenza A virus (A/California/7/2009), a variety of H3N2 influenza A virus (A/Victoria/361/2011), and a variety of influenza B virus (B/Wisconsin/1/2010-Yamagata lineage).
In order to measure this, the CDC performs laboratory testing of patient influenza isolates. According to the antigenic characterization by CDC laboratories at the Flu Surveillance website, approximately 4% have turned out to be H1N1 and matched the vaccine component exactly. Approximately 68% of the virus isolates have turned out to be H3N2, and of those there was greater than 99% match to the vaccine component. The remaining 28% of the laboratory isolates turn out to be influenza B, the most prevalent of which is a component of the seasonal vaccine. This laboratory data argues that the prediction of vaccine components for the seasonal influenza was actually very good this year, as the circulating influenza types are in fact the ones that were predicted to be common.
So given the severity of the seasonal influenza outbreak, and the apparent success in predicting the components of the vaccine, what is the explanation why this year’s flu is so strong? There are several possibilities:
- the vaccine components actually do not provide sufficient protection, due to an as yet unknown reason
- minor varieties of influenza virus not contained in the component vaccine are responsible for significant disease
- people who are getting sick did not receive a vaccine, so actually the vaccine is working well
Randomized control studies indicate that overall the seasonal influenza vaccine offers 50 to 70% efficacy in preventing laboratory diagnosed influenza when the antigens were well matched. In years when the antigens were not well matched, the efficacy has been very low. There is currently very good agreement between the components of the seasonal flu vaccine and observed varieties of influenza virus in clinical specimens, which would argue that the vaccine should be highly effective at offering protection. In order to determine whether an appropriate response to the vaccine has been achieved, patients would need to have antibody titers to the vaccine components determined. This is done in clinical trials prior to distribution of the vaccine, however without further study, there is no way to emphatically determine that the immune response promoted by the vaccine is currently ineffective. The Centers for Disease Control hope to have determinations for vaccine efficacy based on clinical observational studies for the 2012-13 seasonal influenza within the next 5 weeks.
The H3N2 influenza has historically been a more aggressive influenza, resulting in more hospitalizations and more deaths when it has been prevalent. Since standard influenza vaccine efficacy results in only up to 50 to 70% efficacy, the spike in severity this season might instead be attributable simply to the high levels of this virus type in the population, as well as a large pool of individuals (over 50%) who have not been vaccinated at this point in the season. Consequently, the CDC continues to urge individuals to get vaccinated if they have not yet been vaccinated.