We started blogging on public health at the beginning of the 2004 – 2005 flu season, although we didn’t concentrate on flu immediately. We intended to use the public health problem of influenza, a disease that contributes to the death of almost 40,000 US citizens a year, as a lens through which to look at public health. The interest in bird flu and pandemic flu followed naturally. The intervening years saw seasonal influenza outbreaks that were milder than previous years, but this resourceful virus made a comeback in the flu season just concluded. CDC has just summarized the 2007 – 2008 flu season and compared it to the three previous years (1004 onward:
During the 2007–08 influenza season, influenza activity* peaked in mid-February in the United States and was associated with greater mortality and higher rates of hospitalization of children aged 0–4 years, compared with each of the previous three seasons. In the United States, influenza A (H1N1) was the predominant strain early in the season; influenza A (H3N2) viruses increased in circulation in January and predominated overall. While influenza A (H1N1), A (H3N2), and B viruses cocirculated worldwide, influenza A (H1N1) viruses were most commonly reported in Canada, Europe, and Africa, and influenza B viruses were predominant in most Asian countries. This report summarizes influenza activity in the United States and worldwide during the 2007–08 influenza season (September 30, 2007–May 17, 2008). (CDC, Morbidity and Mortality Weekly Reports)
There appeared to be two peaks if you use the proportion of visits to outpatient facilities for acute respiratory disease as a measure, one at the time of the Christmas holidays and the second in mid-February. CDC speculates the earlier peak might have been a result of fewer routine outpatient visits, thus inflating the percentage for acute illness. The later mid-February peak is more in line with previous seasons where flu reaches its height mid-February to early March. In the US, A/H1N1 predominated early in the season, but as flu cases started to pick up speed it was superseded in most regions of the US by A/H3N2 — except in the Mountain and Pacific surveillance regions, a reminder that the dynamics of this virus are complicated and still not well understood. As if to emphasize this, in Canada and Europe influenza A but A/H1N1 predominated, while in Africa and Asia there was more influenza B than influenza A. This was also true of some European countries, and even those with more influenza A than influenza B had quite high proportions of flu B compared to the US.
As previously reported, the circulating flu viruses were not a good match for this year’s flu vaccine. The match for A/H1N1 was 66%, not great but much better than the 28% match for A/H3N2. There was essentially no match for influenza B. Resistance to one or another of the two main classes of antivirals, neuriminidase inhibitors (oral form, Tamiflu, inhalable form, Relenza) and M2 inhibitors (adamantanes) showed an interesting pattern. All influenza A viruses remained sensitive to the inhalable Relenza (zanamivir) but A/H1N1 showed increased resistance to Tamiflu (oseltamivir). The proportion of H1N1 resistant increased slightly from last year, from 0.7% to 10.7%. No H3N2 were resistant to Tamiflu. But virtually all the H3N2 isolates were resistant to the adamantane class of antivirals resistance was at the same level as for Tamiflu (10.7%), but interestingly, there was no overlap, i.e., no isolate was resistant to both Tamiflu and one of the adamantanes. If resistance to the two drugs were completely independent at 10% apiece, one would expect 1% of the 918 H1N1 isolates, or 9, to be susceptible to both. This suggests there might be a biological reason for the lack of overlap. Stay tuned.
Overall these data confirm what we already knew: this was a much worse flu season than the previous two. The proportion of deaths attributed to pneumonia and influenza peaked in mid-March and was higher than previous years. Here’s a time series going back to 2004 so you can see the extent of the differences (Source: CDC, Morbidity and Mortality Weekly Reports):
83 of the deaths were in children, spread out over 33 states. The bad vaccine match may have contributed to the severity, although studies one on subpopulations strongly suggest that the vaccine had partial to good effectiveness for influenza A despite the mismatches.
While the pandemic flu shoe didn’t drop this year, influenza still left a pretty big foot print.