The number of influenza cases this year seems to have peaked and started to drop in the last few weeks, according to both the CDC surveillance data and Google Flu Trends (which updates more in real-time). Does that mean swine-origin influenza virus is gone for good? We don’t know, of course, but I was struck by the resemblance of this year’s caseload to charts I’ve seen of the 1918 influenza mortality rates:
|Weekly influenza cases for 2009 (red) vs. weekly influenza mortality from 1918 (black)|
I don’t have the original data that were used to make the 1918 data1 (black traces; these are mortality data, rather than cases; some sources suggest that this would underestimate the case number for the spring/summer peak of 1918, if the virus increased its virulence before the fall outbreak) , but I think I have the chart aligned with the CDC’s weeklies (red) for this year’s pandemic. 2 They’re not identical, but they’re similar so far. In particular, there was a surge in the summer, a big peak early in the fall — much earlier than standard seasonal influenza, which doesn’t usually get going until January or February — and then the fall cases dropped dramatically. The 1918 influenza was followed by a third, smaller, peak, in winter, around the usual period for influenza.
In 1957, when a new pandemic influenza struck the US, the pattern of three waves — smallish in summer, very large in fall, followed by a slightly smaller but still major wave in early winter — was also broadly similar.3 Again, there was a spring/summer wave throughout the US, starting in June and peaking, maybe, in August.4 The fall outbreak started in September and peaked in late October, there was a lull, and then there was a new surge early in 1958, this time perhaps peaking a little later in the winter than the 1918 version.
|1957/1958 influenza cases, by week5
The next pandemic was 1968/1969, when H3N2 moved in and supplanted the H2N2. The pattern here seems different: There was little or no spring/summer wave,6 and while the outbreak did start a little earlier than usual, it was only by a few weeks:
The first outbreaks in the civilian population developed in Puerto Rico and Alaska in late September and early October. The first outbreak in civilian population in the continental USA did not develop until the third week of October, when the small desert city of Needles, Calif., reported an influenza-like illness involving 35%-40% of the of the population. 7
(By comparison, by the third week in October this year, the pandemic seems to have just about peaked.) Along with the late start in 1968, there was no lull; the influenza kept building to its peak, in December, and then quickly dropped down and stayed down. It was really more like a very large seasonal epidemic, very different from the patterns of 1918, 1957, or 2009. (There’s actually a more impressive bump in the following summer, 1969, so this might be simply because the virus didn’t reach the US in time, and carried over into the summer rather than presaging the pandemic.)
|1968/1969 influenza cases, by week, compared to previous years7|
The next pandemic was 1977-1978, when H1N1 returned. I don’t think we have good data for that, because the main measure was death, and that pandemic strain didn’t impact mortality significantly. 8 ,9 However, as far as I can tell, that was mainly a winter-only wave. So the common claim that pandemics come in several waves is not universally true — just two of the four 20th century pandemics acted that way — but it does seem to be true for the 2009 pandemic.
In any case, based on previous pandemic patterns where the disease started this early, I’m guessing that we’re already past the very worst of the 2009 SOIV outbreak, and we’re going to enter a bit of a pause; but it’s going to come back early in 2010 — perhaps not to the same levels as we’ve been seeing in late October/early November, but not too far off.
- Taubenberger JK, & Morens DM (2006). 1918 Influenza: the mother of all pandemics. Emerging infectious diseases, 12 (1), 15-22 PMID: 16494711[↩]
- This was the introduction of an H2N2 strain that replaced the previously-circulating H1N1 virus[↩]
- I haven’t seen the data for the summer of 1957, so this is based on comments in the descriptive papers, and I don’t know how big the summer wave was[↩]
- D. A. Henderson, Brooke Courtney, Thomas V. Inglesby, Eric Toner, & Jennifer B. Nuzzo (2009). Public Health and Medical Responses to the 1957-58 Influenza Pandemic Biosecurity and Bioterrorism, 7 (3), 1-9[↩]
- You can see a little bump in the summer of 1968, which could conceivably have been the summer wave, but it’s definitely much less dramatic than the 1957 or the 2009 summer.[↩]
- Robert G. Sharrar (1969). National Influenza Experience in the USA, 1968-69 Bull Wld Hlth Org, 41, 361-366[↩][↩]
- Lui, K., & Kendal, A. (1987). Impact of influenza epidemics on mortality in the United States from October 1972 to May 1985. American Journal of Public Health, 77 (6), 712-716 DOI: 10.2105/AJPH.77.6.712[↩]
- The 1977-78 H1N1 was almost certainly a laboratory accident, a release of an earlier strain from pre-1957. Accordingly, most people over their mid-20s were already immune to the pandemic virus. Since older people are usually the ones with the highest mortality rates, the 1977-78 pandemic didn’t trtanslate into increased mortality, and doesn’t show up in mortality rates.[↩]