The 2011-2012 influenza season is late and likely to be light.
The season only officially began at the end of February, marked by the point at which the third consecutive weekly percent of respiratory specimens testing positive for influenza surpassed 10%. This is the latest start to the U.S. flu season in 24 years. According to the Centers for Disease Control and Prevention, the percentage of respiratory samples testing positive for flu has remained below the 10% mark until February only once before, during the 1987-1988 season.
By Dr. Michael E. Pichichero
Clinicians around the country have been seeing essentially no flu among our pediatric patients. But at the same time, we are seeing a prolonged respiratory syncytial virus (RSV) season. Usually we see RSV in the northeast from about December through January, when it’s replaced by flu. This year, RSV has continued through all of February and into March. Because RSV is predominantly a more severe and symptomatic illness in younger children, our population of respiratory illness patients mostly involved visits by children less than 3 years old. Flu, in contrast, results in significant symptomatic illness in all ages of pediatric practice.
The CDC is still recommending that people get vaccinated, but many primary care providers have administered all of the influenza vaccine they ordered months ago. Unless there’s a serious resurgence, it would be unlikely many would order more at this late point in the winter season with spring just around the corner.
We don’t really know why the 1987-1988 season was late, but this time around we have several possible explanations. There was no change in vaccine strains circulating in the 2011-2012 winter season compared to the 2010-2011 season. Since there was no change in influenza strains, patients who were vaccinated last year are still protected this year. In fact, the H1N1 component from the 2009 pandemic has remained the same for three consecutive seasons now, so I think we’re seeing much wider persisting immunity throughout the population.
Add to that patients who missed their vaccine last year but got their flu vaccine this year are protected too, as are patients who were vaccinated last year and this year. The vaccine given this year really boosted their immunity to sky-high levels. Together, it seems we have achieved very high herd immunity. Recall that herd immunity occurs with many vaccines. Herd immunity occurs because vaccinated persons are unable to contract and transmit an infectious disease, so even those who are not vaccinated get protection because there is less circulation of the infectious agent and thereby less transmission (contagion).
There has long been a mistaken belief that immunity to the flu vaccine wanes toward the end of the season. This is not true. The reason that flu vaccines are recommended for annual administration is because the flu strains "drift," resulting in a loss of protection afforded by the vaccine strains included in the previous year’s vaccine. I have been asked by parents whether they should postpone flu vaccination until early winter rather than receive the vaccine in the fall when it becomes available. They think the protection from flu vaccine is short-lived and may not last through the flu season. I explain that there is no need to delay vaccination. Immunity induced by vaccination lasts for years, not months.
(c) Dan Higgins/CDC
Influenza, the ultrastructure of which is graphically represented above, is late and likely to be light this season.
A second factor that may be contributing to the late and light flu season could well be the uptake of the influenza vaccine among all pediatric age groups. Since the CDC and American Academy of Pediatrics recommended universal vaccination for all children down to 6 months of age, the number of vaccinated patients has been steadily rising. This too adds to the herd immunity effect. We don’t have national data yet, but our pediatric practice has been purchasing more vaccine each year since 2009. I suspect this is a national trend of increasing use among all pediatric age groups.
A third factor that influences the flu season is the weather – colder weather. We had a mild winter. Influenza is transmitted through respiratory droplets, mainly by sneezes and coughing. When people remain indoors due to colder weather there is more opportunity for sneezing and coughing to result in respiratory virus transmission. With less indoor crowding, we would expect to see less transmission.
The benefits of a light flu season are many. Fewer flu cases result in fewer cases of otitis media. Part of this could be due to the impact of the new 13-valent pneumococcal conjugate vaccine. However, there is ample evidence that kids get ear infections with flu. I think the decline this year in ear infections is due to both factors.