Dear Dr. Jeff: If I get a flu shot, will it also protect me against the bird flu? -L.E.
Dear L.E.: Probably not?but you should get your flu shot anyway!
As Dr. Gerberding, the Director of the Federal Centers for Disease Control has recently said, pandemic bird flu may well be a "worrisome possibility," but with 13 states already reporting outbreaks of "regular flu," this year's flu season is truly at our doorsteps.
Vaccination remains the single best prevention against a disease which claims over 36,000 lives each year in this country alone.
Avian H5N1 influenza has infected some 100 people worldwide since 2003, and about half of them died. In 1997, a H5N1 outbreak in Hong Kong claimed six lives.
What are the historic and biologic relationships between avian and human flu?
Probably all influenza viruses originated in birds. Wild birds can safely carry a large variety of the viruses without themselves becoming ill. Domesticated birds are far more susceptible. In the case of highly pathogenic subtypes, mortality rates can reach 90 to 100 percent within 48 hours.
Surface proteins on influenza viruses largely determine the virus's infectivity and virulence. Influenza A viruses are subtyped according to their hemagglutinin ("H_") and neuraminidase ("N_") surface proteins. There are 16 known "H" subtypes and nine known "N" subtypes.
All combinations have been found in birds. Infections with several subtypes do occur in humans, although mostly uncommonly and inefficiently. There are currently only three known subtypes that spread widely among humans (H1N1, H2N2, and H3N2). Each fall, one or the other of them causes outbreaks of "regular," seasonal flu.
Historically speaking, though, their initial appearances were far more significant. They were the causes, respectively, of the 1918-19 "Spanish flu," the 1957-58 "Asian flu," and the 1968-69 "Hong Kong flu" pandemics. The global death toll in the 1918-19 pandemic is thought to have reached 50 million people.
How and why do influenza pandemics occur? Influenza viruses mutate frequently and sometimes rapidly. If more than one subtype co-infects the same host, re-assortment of genetic material can occur among them. Re-assortment can create a new "avian" virus, which has gained the ability to spread easily and directly to humans and among humans. This same process can also occur more gradually through adaptive mutation.
According to the CDC, three conditions must be met for a pandemic to occur. First, a new influenza subtype must emerge (to which we have not yet established protective immunity). The new subtype must be capable of infecting humans and of causing serious illness. Third, and very importantly, the new subtype must spread easily among people.
The H5N1 avian influenza virus which has broken out in Asia, and has now spread to Europe, is certainly a new virus for humans. Considering the numbers of birds that have been involved, H5N1 has so far infected very few people (only 100 cases or so). When it has jumped hosts, it has caused very serious illness. In fact, it has remained so lethal that it has not spread efficiently. Transmission has thus far occurred almost exclusively from birds to people. Spread of the virus from one ill person to another has been reported only very rarely, and when it has occurred, it has not continued beyond that second person.
In other words, so far, that critical third step in the genesis of a pandemic has not occurred. Worldwide, concerted efforts are being made to prevent it from ever occurring. Take a look at the World Health Organization and the Centers for Disease Control pandemic flu web pages (http://www.who.int/csr/disease/avian_influenza/en/ and http://www.cdc.gov/flu/pandemic/). Or closer to home, the Maine CDC web site (http://www.maine.gov/dhhs/boh/pandemic_flu_info.htm). Here at Bowdoin, our Campus Emergency Management Team will focus on responding to a pandemic flu outbreak in our annual preparedness exercises in the spring.
In the meantime, L.E., consider this. This year's flu vaccine protects against H3N2 and H1N1 (plus Influenza type B). Maybe the less (human) influenza we all have, the less genetic material will be on hand for rogue (avian) flu to join up with and re-assort. We might be doing our very small part to stop the antigenic drift that would make a virus like H5N1 more infectious to humans. We would certainly also be protecting ourselves against a known and very-soon-to-be-present cause of considerable morbidity and mortality.
We anticipate receiving our next allotment of flu vaccine next week and holding a flu shot clinic the following week.
See you then!
Jeff Benson, MD
Dudley Coe Health Center