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DEAR DR. ROACH: Why is there a flu season instead of it being a year-round health challenge? And why do flu strains originate in Southeast Asia instead of, say, Poughkeepsie, New York? — M.S.

ANSWER: Why there is a flu season is a bit of a mystery. Scientists used to think it was due to people staying indoors more during cold weather or because vitamin D levels are low. But research from tropical countries suggests that it has more to do with the combination of temperature and humidity that is optimal for influenza virus transmission, which likes cooler and drier conditions. In areas with very little variation during the year, there isn’t a strong predictable flu season. There remain sporadic cases of influenza even in North America throughout the year.

Influenza can grow and cause disease in the respiratory systems of not just humans, but also pigs and some birds, such as ducks and chickens. That’s why you hear about “swine flu” and “bird flu.” In some areas of Southeast Asia, all three may live under one roof, which allows the virus to more easily “swap” genetic material and mutate into different strains.

Some of the deadliest influenza epidemics have come when genetic information from one species is transmitted to another. In 1918, for example, it seems that the virus came from birds to humans. Five hundred million people became ill and 50 million died in that epidemic worldwide. The world mostly called it the “Spanish flu,” but the first cases known were actually in the United States — not Poughkeepsie, but at an Army camp in Kansas. Still, it appears it may have started in China, after all, a year previously. Fear of another such epidemic (“pandemic” is used for such an enormous outbreak) keeps epidemiologists — the scientists who study diseases — awake at night, and drives the search for better vaccines to stay ahead of the constantly mutating influenza virus.

DR. ROACH WRITES: Many people, including a pharmaceutical company, have written to question my advice to give any flu vaccine, even those made with eggs, to a person with a history of egg allergy. The prescribing information for FluMist, for example, notes that those with a severe reaction, such as anaphylaxis, should not receive FluMist. However, the Advisory Committee on Immunization Practices and the Centers for Disease Control and Prevention state: “People with a history of egg allergy of any severity should receive any licensed, recommended, and age-appropriate influenza vaccine.” They caution that people with a history of severe allergic reaction to egg — that is, any symptom other than hives, and including anaphylaxis — undergo vaccination by a medical provider who is able to recognize and manage severe allergic conditions.

Taken together, several studies have shown that, in over 4,000 individuals with egg allergy, including more than 500 people with a history of anaphylaxis, the flu shot was given with zero acute systemic allergic reactions. Because of this demonstrated safety of regular flu shots, I feel that making people search for the egg-free options (Flublok and Flucelvax) is unnecessary and may cause some people to forgo vaccination.

Interestingly, last year the egg-free vaccines appeared to have about 20% better efficacy than vaccines made with eggs. My own hospital is distributing an egg-free vaccine this year, so that’s what I got for my vaccine — my 33rd consecutive annual flu shot.

The CDC also notes that “previous severe allergic reaction to flu vaccine, regardless of the component suspected of being responsible for the reaction, is a contraindication to future receipt of the vaccine.”

Finally, I want to re-emphasize that FluMist is indicated only for those ages 2-49.