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Commentary
Initial H1N1 Attack
Rate Raises Pandemic Concerns
Recombinomics
Commentary 22:55
January 22, 2010
Around one child in every three was infected with 2009 pandemic H1N1 in the first wave of infection in regions with a high incidence, ten times more than estimated from clinical surveillance.
The above comments indicate that H1N1 infections in children in England in the first wave were about 10 fold higher than levels reflected in clinical surveillance. However, it is likely that H1N1 infections were even higher than frequencies based on neutralizing antibody levels equal or above 32. Anecdotal reports of lab confirmed re-infections suggest initial infections did not produce protective antibody levels. Similarly, clinical trials in Australia indicated 31% of participants had titers of 40 or higher at baseline and most increased titers by at least four fold after vaccination, further indicating that initial infections did not produce robust titers.
In other countries such as the United States, the frequency in students in hard hit areas was markedly higher than 30-40%. Although schools were instructed to stay open if physically possible, many school districts closed because of absenteeism in faculty and staff. Those that remained open saw absenteeism rates were above 20% for extended periods of time. Since students were instructed to return to class after being fever free for 24 hours, many students were only absent 1-3 days. Moreover many were asymptomatic or without fever, so they were not absent. Consequently schools with double digit daily absenteeism for 1-2 weeks had a likely attack rate that approached 100%.
In many areas which had two waves, the target population of the second wave was somewhat older, suggesting a high percentage of the under 65 population was infected with H1N1. This widespread immunity will put pressure on the virus to grow at higher levels or escape from the immune response, leading to concerns of a more severe upcoming wave. Reports of receptor binding domain changes, especially at position 225 appear almost daily, raising concerns that this sub-clade could be transmitting at a much higher frequency in subsequent outbreaks. In some countries the cases fatality rate for D225G/N is at 100% and today’s weekly epidemiological report indicates that over-all 50% of sequences with D225G are from fatal cases. Recent death clusters in Romania and Nepal raise concerns that the frequency in severe and fatal cases will be markedly higher in the next wave.
More outcome data on sequence characterization sheets would be useful.
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