The novel swine flu virus, H3N2v, which is creating health problems in the USA, was, in fact, transmitted to pigs by humans. The virus underwent certain genetic changes in pigs and is now infecting human beings with renewed vigour, says epidemiologist Dr Danuta M. Skowronski, head of the department of influenza and emerging respiratory pathogens at British Columbia Centre for Disease Control, Canada.
The H3N2v had its origins in a human H3N2 ancestral influenza virus strain, since the 1990s. Dr Skowronski led the team that unravelled the genetic mystery of the latest strain of the swine flu virus. “Human influenza viruses are constantly evolving over time in response to the immune pressure that builds up in the population. This does not happen to the same extent among swine herds because of the short life-span of pigs, which are generally brought to slaughter at an early age,” Dr Skowronski said.
Consequently, swine infl-uenza viruses don’t evolve at the same rate as human influenza viruses. The hu-man H3N2 influenza viruses, now circulating and in-ducing antibody protection in the human population, are thus different from tho-se found in the swine population and from H3N2v. “We see age-related differences in antibody protection in our sero-survey. Young children, who were not alive in the 1990s, lack cross-reactive antibody to H3N2v. Conversely, those in their late adolescence or young adulthood have evidence of immunity to H3N2v — likely to be acquired due to exposure to ancestral human strains of the 1990s back when they were children,” Dr Skowronski said, adding that what is interesting is the decline in cross-reactive antibody to H3N2v in middle-aged and older adults.
She said that the study conducted by her team showed that recent seasonal influenza vaccines do not improve sero-protection against H3N2v. This is because the strains included as current vaccine components are about 15-20 years different antigenically from the viruses of the 1990s. According to Dr Skowronski, the population is not entirely immunologically naive to the novel emerging H3N2v strain — teens and young adults show some protection — and this helps diminish its pandemic potential.
“However, we find that children and older adults show broad susceptibility. That means the risk to the population is not zero and that certain groups may be more susceptible,” she noted. Scientists will now use those measured sero-protection rates to quantify the epidemic risk, that is, the risk of small or large-scale outbreaks. “Our findings also indicate that if there is an epidemic spread of H3N2v, then a specific vaccine would be needed against it. This is because the current seasonal influenza vaccines do not appear to improve cross-protection against H3N2v.”