It’s April in Rhode Island and the long winter cold spell is broken. But one guest to the spring party hasn’t yet arrived. The seasonal flu, which usually peaks about February, is conspicuously absent this year. Did this year’s H1N1 pandemic have something to do with the missing flu season?
One explanation for the lack of seasonal flu is that H1N1 was a “dominant strain” in 2009, meaning that it represented the vast majority of flu cases. Although the phenomenon isn’t fully understood, H1N1 may have crowded out other flu strains by infecting people early in the season, before other strains had a chance to spread.
In many flu seasons, two or more strains of seasonal flu are circulating simultaneously. It is not unusual for someone to be infected with two strains at once (in fact, the two strains can even combine to create a new strain). However, a single dominant strain is fairly common during flu seasons, suggesting that there is some competition between strains that makes a second strain less likely to infect a person or to spread through a population. H1N1 was the dominant strain this year, and seasonal flu strains may have been less likely to spread through the population.
Partial immunity to flu strains may be another factor in play. Adding the number of people who got sick with H1N1 this year to the number of people who received the vaccine, it’s possible that more than half of Rhode Islanders have some form of immunity to the H1N1 virus. There is no data to suggest the H1N1 vaccine or virus gives partial immunity to any seasonal flu strains and the seasonal strains do not seem to be very similar to H1N1. This means that partial immunity was not a significant contributor in blocking the spread of seasonal flu.
It seems unlikely that seasonal flu will strike this year if it has not yet done so by the start of spring. However, the first wave of H1N1 hit the U.S. in late April of last year, so we are not out of the woods yet. Flu pandemics historically come in multiple waves, and a third wave of H1N1 is still a possibility. There have even been reports of some new cases of H1N1 in Connecticut and elsewhere in the country, so if you did not get your vaccine yet, now would be a good time to do so. H1N1 will be one of the three flu strains included in the seasonal flu vaccine for next year, and Rhode Islanders have a good head start if H1N1 remains the dominant strain.
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