# 4369
Since influenza is but one of many flu-like illnesses, and in many cases people can become infected (and potentially transmit) the virus without ever knowing they were sick, post-wave seroprevalence studies are our best tool for quantifying a pandemic.
By taking blood samples from a cross section of society and looking for antibodies specific to a particular virus, researchers can extrapolate with pretty good precision how prevalent the virus was during an outbreak.
While one study, conducted in one community, may not tell us all there is to know about the spread of the virus – when you combine it with other studies around the world – you can begin to piece together a more complete picture.
These studies take time, but already we’ve started to get some early results. Last November we learned in UK: H1N1 Serology Tests Reveal Significant Asymptomatic Rate that while 30% of school kids tested in the UK showed antibodies to the H1N1 virus, only about 10% reported illness.
New Zealand is another country conducting seroprevalence studies (see New Zealand To Conduct Seroprevalence Studies), and those results are due in May or June.
This weekend, in PloS Currents Influenza we get the results of a seroprevalence study conducted at two Pennsylvania Hospitals in late November of last year. This was after the peak of the second pandemic wave, and before the H1N1 vaccine became widely distributed in that region.
Seroprevalence Following the Second Wave of Pandemic 2009 H1N1 Influenza
By Ted Ross, Shanta Zimmer, Don
Burke, Corey Crevar et al (10 authors)
Conclusion
The novel 2009 H1N1 influenza strain has been found to have a relatively low transmissibility, i.e. an R0 [31][32] of 1.3. Our finding of high anti-2009 H1N1seroprevalences among school children and high anti-1918 H1N1 seroprevalences among the elderly suggest that further sustained viral transmission is not likely.
With current estimates of seroprevalence and continued increases in population due to vaccination, a significant change in viral antigens or a change in population immunity would be required for further disease spread. However, we cannot rule out the possibility that geographical pockets of limited immunity may be present in which a third wave may yet occur.
Ongoing viral and serosurveillance efforts will be essential to inform decisions around vaccination and other disease mitigating strategies.
This is just a snippet of a long and informative study, and well worth reading in its entirety. The uneven spread of immunity shown in the Chart in Table 1 is particularly interesting (I’ve captured a portion of it below).
Those aged 10 to 19 years were nearly twice as likely to have developed antibodies as the other age groups. The spike in reactivity among those born in the 1920s probably stems from previous exposure to a similar virus 80 years ago.
The authors suggest that - between pre-existing immunities, acquired immunity through exposure to the virus, and vaccination - a large enough segment of the US population is likely immune to the H1N1 virus as to preclude our seeing a serious 3rd wave of the virus.
And if that sounds vaguely familiar to you, it should.
It is essentially the same conclusion that Professor Ian York, writing on his Mystery Rays blog, came up with a couple of weeks ago in How many Americans are immune to H1N1? .
Of course, there are caveats (there are always caveats).
Viruses mutate and change antigenically over time, and in doing so can evade pre-existing immunity. H1N1 will no doubt do the same.
It may take months, or perhaps years, but if H1N1 sticks around it will eventually change enough to re-infect some of those who today have immunity.
Additionally, countries that have not vaccinated large segments of their population may still be at risk of additional waves.
For all of the negative press regarding `wasteful’ vaccines, the immunization of 70 million Americans has probably had a lot to do with our not seeing a resurgence of the virus.
The HHS deserves immense credit for rolling out a safe and effective vaccine in record time.
While the uptake of the vaccine might not have been as high as they had hoped, it appears to be making a difference.
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