H1N1 rates debated in those vaccinated for seasonal flu

Possible confounding bias surrounds four Canadian studies that saw an increase in H1N1 influenza rates among those vaccinated for seasonal flu.

Investigators of a school outbreak of H1N1 in rural northern British Columbia in late spring 2009 noted those vaccinated against seasonal influenza had higher rates of illness characterized by fever and coughing. They sought to confirm this observation with three case-control investigations in Ontario and Quebec, as well as a transmission study in 47 Quebec households infected by H1N1. Results were reported April 6 in the online journal PLoS Medicine.

Among 1,226 lab-confirmed H1N1 cases and 1,505 controls, odds ratios for H1N1 infection requiring treatment increased between 1.4 and 2.5 in those who received seasonal flu vaccination.

The studies' observational nature makes potential confounding a significant concern, and patients may have altered their patterns of health-care seeking behavior, the authors wrote. But Canada's sentinel analysis protocols would have standardized for that. Accompanying commentary said although confounding plays a major part in interpreting observational studies, researchers fully described its impact in their research.

The commentary also noted that six other studies in the U.S., Australia and Mexico either showed no impact or a protective effect of H1N1 vaccination. Canada's sentinel system will provide more data soon, and “[T]his perplexing experience should teach us best how to react to disparate and conflicting studies and prepare us for the next public health crisis, so that we can better manage future alerts for unexpected risk factors,” the authors noted.