The Sudden Birth of H1N1 “Swine” Flu: What Does the Future Hold?

The past and future flu virus …
In recent weeks, an international outbreak of illness has occurred; it is caused by a new strain of influenza virus (2009 A/H1N1) that contains a combination of swine, avian, and human influenza virus genes. In this post, Anthony L. Komaroff, MD, provides some additional background information.

Pigs, birds, and humans are each susceptible to many flu viruses. Typically, these viruses infect only one species. However, sometimes these viruses swap genes and create new viruses that can infect more than one species. Even then, new viruses that are capable of infecting two species typically are very difficult to transmit from human to human. Sometimes, however, further recombinations or mutations of genes create a virus that can spread rapidly among humans - thus creating a global pandemic. The worst global pandemic in modern times was the flu pandemic of 1918-1919. It affected about a third of the human race and killed at least 40 million people in roughly 1 year - more than have been killed by AIDS in 3 decades.

The World Health Organization and the CDC have confirmed that the new swine flu virus is transmitted between humans. Almost surely, like other flu viruses, it can be transmitted by aerosol and by skin-to-skin contact with an infected person. The new virus is resistant to amantadine and rimantadine but is sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza). Based on experience with other flu viruses, treatment would be most effective if given within 2 days of symptom onset. Obviously, no vaccine is available yet for the new virus, and the CDC has expressed doubt that the current flu vaccine offers much, if any, protection. People should be considered to be contagious until at least 7 days after symptom onset; with children, that time might be as long as 10 to 14 days.

Influenza viruses are transmitted more easily in cold dry air. Even if the current outbreaks in the northern hemisphere quiet down in the next weeks and months, some experts are concerned that the virus will produce epidemics in the southern hemisphere during winter (June through August), and then return with a vengeance to the northern hemisphere in its fall and winter seasons. That’s exactly what happened in the pandemic of 1918-1919. However, that pattern doesn’t always happen: The swine flu outbreak that occurred at Fort Dix, New Jersey, in early 1976 did not return the next winter. However, in anticipation that it would, 40 million people were immunized, and about 1 in 100,000 vaccinated people developed Guillain-Barré syndrome.

If a vaccine against the current H1N1 virus is ready in the fall, government officials will face several difficult decisions, without much data to predict the future. First, they will have to decide whether to recommend widespread immunization against 2009 A/H1N1, even if no evidence of its resurgence is available - and without knowing whether a vaccine against this new swine virus would also provoke rare cases of Guillain-Barré syndrome. Second, policy makers will have to decide whether to recommend immunization against other circulating strains of human influenza virus, as well.

Anthony L. Komaroff, MD