[Disclaimer: We're featuring the following study and estimate as interesting food for thought. We can't vouch for their accuracy; you would need to do your own reporting to evaluate that.]
Two different assessments of U.S. immunity to H1N1 have emerged recently. The first suggests the number of Americans infected by swine flu in 2009 (roughly 63 million). The second estimates the number of Americans who currently have immunity to H1N1 (somewhere roughly between 150 and 165 million).
Assessment #1: A new study published in PLoS Currents: Influenza estimates the number of Americans who were infected with H1N1 in 2009. (Read our post about PLoS — the Public Library of Science — here.) The lead author is Ted Ross, an associate professor of microbiology and molecular genetics at the University of Pittsburgh.
The study looks at levels of antibodies to 2009 H1N1 in Pittsburgh-area residents. (Infection with a virus stimulates antibody production, which then confers immunity.) It examines blood from “846 persons that ranged in age from 1 month to 90 years of age.” The samples were taken from “hospital and clinic patients in mid-November and early December 2009.”
It’s possible that some people with antibodies to 2009 H1N1 got them from the vaccine rather than infection with the virus — but “the timing of the sampling relative to vaccine availability in Pittsburgh suggests that these samples are likely from a largely unvaccinated population during the peak of the second pandemic wave.” In other words: the data probably approximate the number of people actually infected by H1N1 in 2009.
Here’s the study conclusion in a nutshell:
21% of persons in the Pittsburgh area had become infected and developed immunity. Extrapolating to the entire US population, we estimate that at least 63 million persons became infected in 2009. As was observed among clinical cases, this sero-epidemiological study revealed highest infection rates among school-age children.
Assessment #2: Ian York, an assistant professor of microbiology and molecular genetics at Michigan State University, recently offered his best educated guess of the number of Americans now immune to H1N1 (on his blog Mystery Rays from Outer Space). As he puts it, there are three ways in which someone could have acquired immunity:
They could have been exposed to a related virus, some time in the past, and have developed a long-term immunity. They could have been infected with [H1N1], somewhere in the first or second wave. Or, of course, they could have been vaccinated.
He collects the available data for each of those categories — emphasizing that “[t]hey’re more or less approximate” — and concludes that:
- Over half the US population as a whole is now immune to the new [H1N1].
- As many as three-quarters of the elderly and two-thirds of the children — the most important populations as far as flu is concerned — may be immune.
- Between a third and about half of this immunity was due to vaccination.
To find York’s full table of high and low estimates — broken down by age group — click here.
Why is it useful to know what percentage of the population may have immunity to H1N1? As the PLoS study puts it, it “provides valuable information about the likelihood of a possible third wave and may be useful in decision-making about immunization strategies.” Or as York writes, the “level of immunity” that he calculated “is probably enough to impact virus transmission drastically.”