The novel swine flu virus, H3N2v, which is creating health problems in the USA, was in fact contributed to pigs by humans. The virus underwent certain genetic changes in pigs and is now infecting human beings with renewed vigour

By Syed Akbar
Hyderabad: The novel swine flu virus, H3N2v, which is creating
health problems in the USA, was in fact contributed to pigs by humans.
The virus underwent certain genetic changes in pigs and is now
infecting human beings with renewed vigour.

According to epidemiologist Dr Danuta M Skowronski, the H3N2v virus
had its origins in a human H3N2 ancestral influenza virus strain from
the 1990s. Dr Danuta is head of influenza and emerging respiratory
pathogens department at British Columbia Centre for Disease Control,
Canada. She led the team that has unraveled the genetic mystery of the
latest strain of swine flu virus.

Danuta told this correspondent that the virus went to pig from man in
1990s and after getting "stuck in time" antigenically for a little
over a decade it is re-infecting human beings. “Human influenza
viruses are constantly evolving over time in response to 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,” she said.

Consequently, swine influenza viruses do not evolve at the same rate
as human influenza viruses. The human H3N2 influenza viruses now
circulating and inducing antibody protection in the human population
are thus very different from those found in the swine population and
from H3N2v which is now re-emerging from swine back into segments of
the human population that lack antibody to it.

“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 who are now in
late adolescence or young adulthood have evidence of immunity to H3N2v
- likely acquired because they were exposed to ancestral human strains
of the 1990s back when they were children during that period,” Dr
Danuta said, adding that what is interesting is the decline in
cross-reactive antibody to H3N2v in middle-age and older adults.

She said 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 her, the population is not entirely immunologically naive
to the novel emerging H3N2v strain - teens and young adults show some
protection - and this likely helps to 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.”

Scientists will now use those measured sero-protection rates to
quantify the epidemic risk - that is, the risk of small or large-scale
outbreaks because of recent clusters. “Our findings also indicate that
if there is epidemic spread of H3N2v, then a specific vaccine would be
needed against it because current seasonal influenza vaccines do not
appear to improve cross-protection against H3N2v.”