Last year’s flu season was bad. Hopefully this year’s will be better (for us, not for the virus). The CDC is changing the recommendations a bit to improve the population’s coverage, and I’m hoping I won’t be quite as busy this winter.
Last year, I provided you with weekly flu activity updates. I’ll probably do that again, but I think we need to kick off the season with an influenza primer. Get ready for some science! (BTW, for a more extensive look at influenza biology, see Effect Measure.)
The Disease
The flu is misunderstood. It is a respiratory illness, not a “stomach bug”. Is is passed through contact with respiratory secretions, such as sneezing and coughing. Infection leads to a sudden onset to fever, muscle pain, and fatigue. Often, there is a dry cough, runny nose, and watery eyes. People with the flu look and feel miserable.
Influenza is a dangerous bug, responsible for around 35,000 deaths yearly in the U.S. This number is arrived at by statistical analysis of excess deaths due to influenza and pneumonia during the winter months. Flu itself sometimes kills people, but influenza is often complicated by bacterial pneumonia. Flu shots prevent the flu, but can also make a flu less dangerous should you catch it anyway. Flu shots are a “very good thing” (but not a sure thing).
The Bug
Influenza is a virus that affects many different animals, including humans. In humans, it occurs as seasonal epidemics, and occasionally global pandemics. Unlike smallpox, polio, and other viruses for which a single vaccine formula is preventative, influenza prevention requires the development of a unique vaccine every season. This is where biology comes in handy.
One way our bodies recognize viral invaders is by the chemicals displayed on their surfaces. In the case of influenza, the two most important surface chemicals are neuraminidase (NA) and hemagglutinin (HA). Small changes occur in the RNA of flu viruses over time, leading to changes in the surface molecules. Even small changes in these surface molecules can change our immune system’s ability to respond to an infection. This process of small changes is called “antigenic drift”, and is responsible for yearly influenza epidemics. (Any molecule recognized by the immune system is called an “antigen”, and the flu surface molecules are important antigens.) The slightly changed virus is more successful than other flu viruses, and becomes dominant during that season (evolution in action!).
Sometimes, flu virus antigens undergo major changes that make them entirely unrecognizable to the human immune system. This is called “antigenic shift” and involves a big change in the flu’s surface molecules. One way this can happen is when a human and animal flu virus co-infect a host and share genetic information. These antigenic shifts are responsible for flu pandemics, as the antigens are new to human populations and spread very easily.
Preparing for antigenic shifts isn’t so easy, but since antigenic drift is happening continuously, it can be tracked and documented. Epidemiologists track the flu across the globe to see what strains are likely to cause trouble during flu season, and use that information to develop the yearly vaccine. The vaccine usually contains three antigens that are predicted to be prevalent in the upcoming season, and even if the coverage is partial, it can reduce the number and severity of cases.
So get ready ‘cuz here it comes.
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