Here is an update on the global swine flu situation as of 4 May 2009, and comments on interesting unresolved questions.
There have been laboratory confirmed cases of infection in over half of the United States (30), with a total of 226 cases and the one death in Texas last week. Globally, 20 countries have reported 985 cases of infection. The highest numbers are in Mexico, with 590 cases and 25 deaths.
According to CDC, very few of the American cases are in individuals who are over 50 years of age. This observation suggests that exposure to previous H1N1 strains might confer some protection against infection, as suggested by Dr. Peter Palese. Individuals who are over 50 today were born in 1959 or earlier. Recall that from 1918 to 1957, H1N1 influenza viruses circulated globally. Those who are older than 50 were likely to have been infected with H1N1 influenza virus strains during those years. Whether or not there is any protection from such previous exposure will likely be determined by laboratory tests in the near future.
Also very interesting is the report from Alberta, Canada that pigs have been infected with the human H1N1 virus. It is believed that the animals were exposed to the virus by a Canadian who had returned from Mexico with flu-like symptoms. This report raises more questions than it answers. But if the new H1N1 virus can readily infect pigs, this could serve as a new source of the virus for future outbreaks. Promedmail poses the tough but important questions:
First, how was directionality of infection established? Was the worker sick when he came in contact with pigs? If so, what lapse in biosecurity allowed a sick human worker to even be on a swine farm as standard biosecurity practices on progressive or up to date swine farms would screen such an individual out and prevent him or her from coming into contact with pigs? Has the worker tested positive for the novel influenza A H1N1 virus? What is the prevalence of the new virus in the swine herd and finally, but most importantly, what quarantine and traceback procedures are in place to make sure that the swine herd does not infect other swine farms? Finally, although we know animal diagnostic laboratories have never seen this virus before in pigs, what surveillance efforts are being made to look at previous swine serum banks or test apparently healthy swine herds on a population basis to actively ensure swine populations are free of this novel influenza A H1N1 virus?
Another important question concerns influenza vaccines. According to Health and Human Services Secretary Kathleen Sebelius, vaccines for the new strains, as well as last season’s, will be ready this fall. Clearly we need a vaccine containing the new H1N1 strain and the relevant influenza B virus strain. But what about last season’s strains – there were two, an H1N1 and an H3N2. If these were also included, that would make for a tetravalent influenza virus vaccine.
Here is one bit of information which would argue against including last year’s H3N2 and H1N1 strains in this fall’s vaccine. In each previous pandemic, the newly emerging viruses replaced the previous circulating strain. For example, in 1968, the H3N2 virus replaced H2N2 viruses that had been causing influenza since 1957. Whether or not the new H1N1 viruses replace the two previous strains will be an important question to address in the coming months. Such information will be used to make decision about the components of the influenza virus vaccine used this fall.
Update: terrific map from WHO on laboratory confirmed cases.