Of the many questions that I receive about influenza, one of the most common is “when will swine flu return to the US?”. In other words, when will the 2009 pandemic H1N1 strain, first recognized in humans in April 2009, emerge for the 2009-2010 influenza season?
“Return” is probably not the correct word to use, because the 2009 swine-origin H1N1 virus has never left the US. The latest statistics published by CDC show that for week 34, 1,109 of 6,410 specimens tested for influenza were positive for the pandemic H1N1 strain. As shown in the graph below, the number of isolations of swine-origin H1N1 virus peaked at week 24 and declined thereafter. Since week 32, the number of isolations of that virus has remained constant.
The decline in the number of virus isolations is probably are a consequence of population immunity and other factors like temperature and humidity that may regulate transmission.
There have likely been many more infections with the 2009 pandemic H1N1 strain than have been reported by the CDC. Together with the relatively constant isolation of the virus since week 32, this suggests that the onset of school in the US – which brings individuals together in large numbers across the country for the first time since June – will trigger the next wave of infections. It has already been reported that influenza activity is rising in the southeastern region of the US, where school began earlier than in the rest of the country.
Seasonal influenza typically does not increase until later in the fall in the northern hemisphere. This timing allows immunization of a good portion of the population with a vaccine that is released at the end of August. A vaccine against the 2009 pandemic H1N1 strain will not be available until October. Until then, there will be many infections with the new strain. The best way to prevent influenza is by immunization, but common sense and good hygiene should be part of everyone’s preventative plan.