A novel strain of H1N1 swine influenza virus has now also been isolated from cases of respiratory disease in Mexico City, New York City, and two locations in Canada. Clearly I was wrong in hypothesizing that the five mild cases in Texas and California reported last week were not a cause for alarm.
In Mexico City there have been over 1,000 cases of respiratory illness, with 86 fatalities. The influenza virus isolated from these individuals is similar to the H1N1 virus found in the American children last week. This virus has caused four cases of influenza in Nova Scotia, and two in British Columbia, Canada. It has also been isolated in Kansas and New York City. All American and Canadian infections have so far been mild.
There isn’t much information yet available about the genetic and antigenic composition of the virus, but CDC has said that it contains genetic material from North American swine and avian influenza viruses, human influenza virus, and swine influenza viruses found in Asia and Europe. The specific combination of viral RNA segments has not been observed previously.
The extent of spread of the swine virus has sent news organizations, governments, and health agencies into a full-tilt pandemic preparedness mode. It’s certainly advisable to be cautious when dealing with a potentially lethal virus, but is it likely that this is the next pandemic strain?
The influenza season is nearly over in the northern hemisphere – it usually does not continue beyond May. Increasing temperature and humidity are likely to curtail transmission of the virus very rapidly. The same virus could return in the fall, but by then a vaccine could be produced and distributed.
The southern hemisphere is another story – the influenza season there is just starting. It is certainly possible that this swine virus might cause extensive epidemics.
Remember that the virus that is scaring everyone is of the H1 subtype. One of the currently circulating human influenza viruses is also of the H1 subtype – the human H1N1 virus. Although the swine H1N1 virus is antigenically different from the human H1N1 virus, it is possible that those who have been infected with the human strain could be partially protected from the swine strain. Such individuals might experience less severe disease than those with no immunological memory of an H1 virus. I have not seen immunological data that would allow assessment of this possibility.
Pandemic influenza has always been a consequence of viruses of a new subtype – for example the 1968 pandemic was caused by a virus of the H3N2 subtype, which replaced the H2N2 virus. The swine virus is of the same subtype as the currently circulating human H1N1 strain. Of course, if the swine H1 HA protein is sufficiently different from the human H1 HA protein it could lead to a pandemic despite being of the same subtype.
Infection of a Spanish woman with a swine H1N1 virus was reported in February of 2009. That virus appears to be phylogenetically close to European swine influenza viruses. Such infections have been reported from time to time, and most likely represent dead-end transmission of a pig virus to a human. The H1N1 swine influenza virus circulating in the Americas has acquired the ability to be transmitted among humans. Within the next few weeks we should learn whether it has the capability of spreading throughout the entire southern hemisphere.
Van Reeth, K, & Nicoll, A (2009). A human case of swine influenza virus infection in Europe – Implications for human health and research Eurosurveillance, 14 (7), 19124-19125