This year's flu season

by revere, cross-posted from Effect Measure

This year's flu season isn't over, but it's almost over, and it was fairly typical and much better than last year, which was nasty. It began at the end of September but didn't take off until early January, peaking in mid Februrary. New cases are still appearing but much less frequently and they are mainly influenza B, which tends to be milder than influenza A. Most of flu/A this year was also of the milder H1N1 sort, which probably contributed to the better outcomes. Here's where we are (source for all charts here):

Comparing this year with previous years shows it to be relatively typical of recent mild flu seasons and also shows how bad last year was:

If we look at it by age group we see again that this was much like three of the past four years except in the older age groups, where it was somewhat better than any of them:

This year's flu vaccine got both the circulating H1N1 and H3N2 strains right (A/Brisbane) but missed flu/B, only 20% of which were vaccine strain (B/Yamagata) while the other 80% were B/Florida. Based on current surveillance data, next year's vaccine will be unchanged for flu/A (A/Brisbane) but changed for flu/B (B/Victoria). Let's hope they get it right. Almost all of the predominant H1N1 was oseltamivir (Tamiflu) resistant but none of the H3N2 were resistant. None of the H1N1 or H3N2 were resistant to the zanamivir (Relenza). The oseltamivir flu/A picture was reversed for the older adamantane antivirals (which work by a different mechanism). Almost all of the H1N1 and none of the H3N2 were sensitive. No flu/A virus was simultaneously resistant to oseltamivir and an adamantane antiviral. All flu/B viruses were sensitive to both oseltamivir and zanamivir (the adamantanes are not effective for flu/B).

Did influenza vaccination make a difference? CDC certainly thinks so. Since seasons where A/H1N1 and flu/B are the predominant circulating viruses tend to be milder, it's not possible to say that getting the flu/A strains right is what made this season milder than last year's flu/A mismatch. Pediatric flu mortality has also been better this year (45 deaths):

But of the 36 deaths in ages for whom vaccination was recommended (older than 6 months), 5 had been vaccinated, although the data are provisional. No data are given for vaccination rates for comparison. It's clear that if vaccination is protective, it is not completely so. Bacterial co-infection (primarily with Staph) was mostly in older children (over age 10).

The workings of this enigmatic virus continue to defy us -- and sicken and kill us on a regular basis. Meanwhile A/H5N1 is still out there, mainly in poultry and wild birds but continuing to infect and kill humans. Each flu season we wait for the other shoe to drop. So far it wasn't the year, [although a pandemic can begin at any time of year]. If we are lucky, it won't ever happen. But better to be prepared than to trust to luck. The cost of being unlucky and unprepared is frightful.