It is becoming conventional wisdom that the 2009 H1N1 pandemic was not as severe as a bad seasonal flu year. That might be true, although I don’t find it much comfort because a bad seasonal flu year is no less bad for being more familiar. But I am not yet willing to assent to the conventional wisdom yet. I don’t think we have had sufficient time to collate all the information that would enable us to make that kind of judgment, which sometimes takes years to evaluate. However bad it was or wasn’t, the pandemic flu strain could kill you just as dead as any other flu. CDC has just released updated information on the risk factors that seem to make it more likely you will wind up severely ill and hospitalized. It confirms what we knew, that certain pre-existing medical conditions up the risk substantially and some of them are extremely common. The analysis was done for both children and for adults, and there are similarities and differences.
The data come primarily from the Emerging Infections Program (EIP), which we’ve featured quite often here (for one example, see here). EIP counts laboratory confirmed cases of influenza in children and adults in 60 plus counties that include 12 metropolitan areas in 10 states (San Francisco CA, Denver CO, New Haven CT, Atlanta GA, Baltimore MD, Minneapolis/St. Paul MN, Albuquerque NM, Las Cruces, NM, Albany NY, Rochester NY, Portland OR, and Nashville TN). The surveillance system uses laboratory and admission databases and infection control logs generated by routine hospital operations. A special compilation of underlying medical conditions is then obtained by chart review.
The results for EIP data collected between April 15, 2009 (when the first swine flu cases in the US were detected) and February 16, 2010 and show that 85% of adults hospitalized with 2009 pandemic H1N1 flu and 58% of children had underlying medical conditions. This means that 1 in 7 flu hospitalizations were healthy adults but almost half of all children. The idea that the only kids who went sour with flu were the chronically ill is unfounded. And for both adults and children the most common “underlying medical condition” is quite common: asthma. We’re not talking about uncontrolled ashtma. Just having asthma and having it mentioned in the medical record is enough to put you in the “underlying medical condition” category. And 9% of the hospitalizations were pregnant women.
Here are the charts for adults and children. Note that a person can have more than one condition, so the percentages add up to over 100%:
WHO has not declared the end of the pandemic, since community transmission is still occurring in West Africa and perhaps elsewhere. Moreover the flu season will be ramping up again in the Southern hemisphere and no one really knows what form it will take. We are in uncharted territory in many ways.
So far there has been no third wave nor any sign of the former seasonal influenza strains. Let’s hope that’s the way it will stay, at least for this winter-spring season. If it does, that will be good but we won’t know why it came out that way. Just that it did.
No one who knows flu is particularly surprised because no one who knows flu would or should be surprised by however it comes out. With flu, it seems almost anything is possible.