Low rates of immunity in adults behind H1N1 resurgence.

The 2009 pandemic strain of H1N1 influenza was back in Canada this year, despite an effective vaccine and very little mutation in the virus, researchers say. Instead, they point to a low level of seroprotection in adults aged 40 to 65 as the likely culprit. Lack of previous exposure and of vaccination have probably combined to create a “pocket of residual susceptibility,” says Dr. Danuta Skowronski, epidemiology lead for influenza and emerging respiratory pathogens at the BC Centre for Disease Control. Skowronski and colleagues at The Canadian Primary Care Sentinel Surveillance Network published the findings earlier this month in Eurosurveillance and presented them at a World Health Organization (WHO) meeting on influenza vaccine in Geneva, Switzerland, Feb. 20. An estimated 90% of influenza strains detected in Canada in the 2013/ 14 season to date are H1N1, according to the study, which used a case–control design to analyze 792 isolates from patients with respiratory illness presenting at sentinel surveillance sites in the five most populous provinces. Skowronski says she was “pleasantly surprised” that this year’s flu vaccine was 75% effective, according to the study’s calculations. “I presented these results to the vaccine strain committee of the WHO, and they have recommended that the same vaccine be retained.” She was also surprised by the viral analysis, which showed that “both antigenically and genetically the virus is well-conserved. It hasn’t changed despite having circulated around the world since 2009.” Why then, has H1N1 come back, although at levels well below those in 2009/10 flu season? A serologic survey conducted last summer in British Columbia (a follow-up to a similar survey conducted after the pandemic in 2010 and published in CMAJ) shows a U-shaped protection pattern by age. “We have found high rates of protection in those over 80 years of age, even before the pandemic,” thanks to their exposure to a similar virus that circulated in the 1930s, says Skowronski. As well, children have high seroprotection rates, probably as a result of high rates of exposure and vaccination in 2009. But middle-aged adults have only a 30% seroprotection rate. “We don’t know the vaccination or infection history” of patients in the serologic survey, explains Skowronski, but she thinks that the virus is now “working its way through adult contact networks.” However, rates of hospitalization and death due to H1N1 this season “pale in comparison with 2009,” Skowronski hastens to point out, as a result of greater overall immunity in the population. Patients hospitalized this season tend to have underlying chronic conditions, including obesity, as a risk factor. “These patients are disproportionately affected,” she says, “We want to get our immunization message to them.” — Carolyn Brown, Ottawa, Ont.