Flu and asymptomatic infections

I’ve mentioned many times on here reservations I have over the current avian flu numbers–how many subclinical or mild infections are being missed? Are they indeed offset by the number of serious disease cases we’re also missing? There’s a reason for these questions, and it’s now out in electronic form in Clinical Infectious Diseases.

A bit of background. I work in Iowa as part of the Center for Emerging Infectious Diseases. One of our pet pathogens happens to be influenza virus, and we have ongoing studies looking at serological evidence of prior infection with swine and avian viruses in people who are exposed to these animals. This current paper looks at a cross-section of people who are in several of these groups: swine farmers, veterinarians, and meat-processing workers. The results:


Using multivariable proportional odds modeling, all 3 exposed study groups demonstrated markedly elevated titers against the H1N1 and H1N2 swine influenza virus isolates, compared with control subjects. Farmers had the strongest indication of exposure to swine H1N1 virus infection (odds ratio [OR], 35.3; 95% confidence interval [CI], 7.7-161.8), followed by veterinarians (OR, 17.8; 95% CI, 3.8-82.7), and meat processing workers (OR, 6.5; 95% CI, 1.4-29.5). Similarly, farmers had the highest odds for exposure to swine H1N2 virus (OR, 13.8; 95% CI, 5.4-35.4), followed by veterinarians (OR, 9.5; 95% CI, 3.6-24.6) and meat processing workers (OR, 2.7; 95% CI, 1.1-6.7).

To boil it down a bit, farmers were about 35 times more likely than the control group to react positively to the swine H1N1 virus used–indicating they’d been previously exposed. Similarly for the swine H1N2 virus–13 times more likely. It appears there is a good deal of zoonotic transmission occurring in this population–it’s just not manifesting with serious clinical symptoms, so we’re missing much of it.

This has implications for what’s going on in Asia with H5N1 as well. How many sub-clincial or mild infections are there with that virus? We don’t know yet–we need good surveillance and seroprevalence studies to address that question. Do these milder infections allow the virus more time to adapt to humans, so that human-to-human spead will become more efficient? We don’t know. It’s largely a black box at this point. It also is something that’s largely been ignored at the response level–do we vaccinate and/or treat these groups early, along with first responders and others? Will doing so prevent a significant number of new infections, protecting not only the human population, but also domestic animals? During the 1918 pandemic, pigs also were susceptible to the virus, and suffered high mortality rates, but this isn’t something that’s garnered much discussion in the current pandemic plan.

This is just a pilot study, and limitations are discussed in the paper. A prospective study is ongoing, which should be able to pick up seroconversion events. Still, even though this is a small study, it hopefully will be food for thought, and will draw attention to including agriculture workers and others in contact with animals in pandemic response plans.

Edited to add other media coverage:
Des Moines Register
Cedar Rapids Gazette
NIAID press release
IDSA news release