New swine flu strain unlikely to cause pandemic

Vancouver, BC – An emerging swine flu strain responsible for hundreds of infections in the United States appears unlikely to trigger a global pandemic in the short term but requires ongoing monitoring, according to a new Michael Smith Foundation for Health Research(MSFHR)-supported study in the journal PLoS One.

To determine the imminent pandemic threat posed by the novel H3N2v strain, researchers at the BC Centre for Disease Control combined estimates of pre-existing H3N2v immunity in the general population with a “contact network” transmission model. This unique model estimated that, should an H3N2v epidemic occur, only about six per cent of the population is likely to become infected – at the low end of the rate typically expected for regular seasonal influenza.

In contrast, using the same methods, the model projected an infection rate of 30 per cent for the novel H1N1 strain responsible for the 2009 global flu pandemic – very similar to what was actually recorded in the population.

H3N2v has been linked to more than 300 US flu cases in the latter half of 2012. No cases have yet been reported in Canada, and the study’s findings suggest broad spread of the virus is currently unlikely.

One reason for the low risk of an epidemic is the pre-existing H3N2v immunity found among certain age groups. An earlier MSFHR-funded study, led by BCCDC epidemiologist Dr. Danuta Skowronski, analyzed antibodies from more than 1,000 individuals and found 25 per cent of the population carries immunity to H3N2v, including 50 per cent of those between the ages of 14 and 40.

By comparison, the same methods found nearly all age groups except the very old were susceptible to H1N1 at the time of the 2009 epidemic.

Childhood exposure plays a key role in priming the immune system to produce antibodies to the various influenza strains. Individuals with H3N2v immunity were likely exposed during childhood to related strains that entered the swine population in the mid 1990s and are now re-entering circulation among humans. Likewise, many elderly individuals carry antibodies that protect against the H1N1-related strains that circulated much earlier in the 20th century.

The BCCDC researchers caution that shifting age demographics can alter the level of immunity in the general population over time. While the risk of an H3N2v pandemic is currently low, this may change as growing numbers of children who lack immunity mature into adolescence and adulthood. The researchers recommend ongoing surveillance to monitor trends in immunity that can help predict future epidemics.