In the spring and summer of 2009, pandemic H1N1 began to spread around the world. While many died and even more were infected,
a global mass mortality scenario, last seen in 1918, failed to materialize.
What happened? Were we saved by our own ingenuity or by sheer luck? Now that the second wave of infection is well behind us,
several studies, and the opinions of experts, suggest it was a bit of both.
“We were very fortunate. We were presented with the best-case scenario by Mother Nature,” says Upton Allen, Chief, Division
of Infectious Diseases at Toronto’s Hospital for Sick Children (SickKids). “It started as a mild infection during the spring,
burned out during the summer while we prepared a vaccine and then implemented mass vaccinations programs for the second wave
of infection by the fall.”
In Dr. Allen’s view, it was not just the favourable time-line that helped the planet dodge a viral bullet: other random factors
provided significant advantages to scientists and public health officials, and ultimately everyone else. Mexico, the location
of the first outbreak of infection, belongs to a robust global disease surveillance system. This meant that the pandemic was
closely monitored from the beginning. This provided excellent information about the way the virus was mutating, which guided
the approach to formulation of the vaccine.
The lucky streak by no means ended there. “The virus also did not mutate into a strain that rendered either the vaccine or
antiviral drugs ineffective,” adds Dr. Allen. “If we had asked Mother Nature, we couldn’t have been given a better scenario.
That might not be what happens next time around.”
Not just luck
On the human ingenuity side of the ledger, two things that public health agencies can take credit for are communications and
vaccination rates. Despite the complex nature of some of the specifics of the messages – who gets vaccinated, with what, and
when – the main message encouraging vaccination came through. “It varied depending on the community, but near the end of December and into January, vaccination rates ranged from percentages
in the high 20s to those in the high 50s,” says Dr. Allen. “This is less than we had wanted for Canada, but it is relatively
good compared to other countries.” For comparison, the Centers for Disease Control in the US estimates 24% of all Americans
were vaccinated.
While vaccination rates were not near 100%, it is not necessary to achieve that level to provide what is called herd immunity.
“For many years, we’ve assumed, and to some extent known, that when you vaccinate a susceptible population, you don’t need
to vaccinate everyone to get the protection,” says Dr. Allen, explaining that once a certain level of vaccination is reached,
the unvaccinated also benefit because they are less frequently exposed to infected people. “Say, within a family. When some
are vaccinated, it reduces the chances other family members are exposed to the virus.”
The Hutterite Communities Study
Recently, this hypothesis was put to an experimental test. Using 49 remote Hutterite farming communities, researchers from
Canadian and American hospitals set out to demonstrate whether herd immunity can be achieved by vaccinating only children.
In half the communities, all children between the ages of three and 15 received flu shots. In the other 24 communities, children
were not given the flu shot. Hutterites tend to live communally, for example eating in the same mess hall, and have little
contact with the outside world. When subjects were tested months later, more than 10% of adults from non-vaccinated communities
had flu. Less than 5% of adults from child-vaccinated communities were infected.
“It’s a useful study. We have long assumed that young children are a vector, so by targeting children one might interrupt
the dissemination of the virus throughout the community,” says Dr. Allen. A vector is anything that transmits a disease. For example, during the Black Death plague of 1348, the vector was rats carrying infected
insects from place to place. In more recent years, epidemiologists, who study the transmission of disease within a population,
have identified "super-spreaders," individuals who account for a disproportionate amount of infection spreading. Children
often meet the criteria for super-spreaders; anyone who has spent time with toddlers in a day care will not be surprised by
this news. According to Dr. Allen, the Hutterite study “confirms there is a benefit to the whole by vaccinating children.”
But did the herd effect occur in Canada during the H1N1 outbreaks? Although it is difficult to come to a definitive conclusion,
Dr. Allen believes that especially in communities that had higher rates of immunization, herd immunity did occur.
What comes next with H1N1?
“The first point is that the pandemic is not over. There is clearly evidence we are over the peak of the second wave but H1N1
is still around. In the Southern USA there is still widespread activity,” says Dr. Allen. He explains that because of low
vaccination rates in some communities, “significant parts of the population are still susceptible, which could lead to a third
wave, though probably not as big as the second wave. If that occurs, it will happen between now and mid-June to mirror what
happened during the first wave.”
But how susceptible a population is to further infection with H1N1 is not just a matter of vaccination rates. Those who were
infected during the first two waves are much more likely to be immune as well. Just how many people were infected is difficult
to determine, because most people only had mild infections and did not access health services. However, Dr. Allen points to
a recent CDC estimate using mathematical modeling that suggests for each reported case, 140 people were infected. “This is
a good thing because it means that the portion of susceptible individuals is much lower than what we would estimate from the
vaccination numbers,” he says, but cautions, “they could be wrong.”
Which is reason enough to get vaccinated if you have not already done so.
