The Medical Minute: The trouble with the flu

By John Messmer, M.D.
Penn State Milton S. Hershey Medical Center

On Thanksgiving, the Associated Press reported that four children died of the flu in Colorado. Although children’s death rates from influenza are usually less than 1 in 100,000 compared to almost 100 in 100,000 in people over 65, children under 1 are hospitalized for the disease just as frequently. Many in the medical community were surprised by the deaths because they came so early in the season and the victims were so young.

The Centers for Disease Control have been monitoring cases of influenza to determine if this year’s vaccine will protect us. The good news is that the current vaccine should work, although the protection might not be perfect. There appears to be a slight difference in the most common strain so far compared to the vaccine being used this year. Why is there such concern over the strain of the flu and getting so many people immunized; and why is the flu mild one year and severe another with epidemics from time to time?

How about a quick biology lesson? Influenza, as it relates to humans, comes in two types: A and B. These vary genetically in the same way dogs and wolves have genetic similarities and differences. Their genetic code is carried on RNA rather than DNA. Flu virus RNA is covered by a membrane that has spike-like projections that contain two enzymes called neuraminidase (N) and hemagglutinin (H). Influenza A has major variations in the type of H and N enzymes on its surface while type B has less variation.

It’s pretty scientific stuff, but although understanding these structures is technical, it helps to make clear why the medical community tracks the disease so closely. Hemagglutinin helps the virus attach to cells in our respiratory tracts and enter them. In our cells, the virus’s RNA makes millions of copies of itself. The newly created viruses manufacture new membranes and are released from the cell to infect other cells. Neuraminidase prevents the viruses from sticking to each other and to the cell in which they grew, a result that would inhibit its ability to spread through the infected host.

Flu viruses have 15 types of H and 9 types of N. Most are found in strains that infect birds and other animals. In humans, H1, H2 and H3 plus N1 and N2 in various combinations cause most of our flu. Occasional outbreaks of avian and swine flu have occurred but tend to be limited in scope.

Influenza strains also have names: Hong Kong, Russian, Asian, Panama among others. These names are based on tiny changes in H or N called “drift.” When the composition changes enough that previously formed antibodies do not attack it as well, it is named for the place and year in which the particular strain of virus is isolated and given a lab number. It is then characterized by its H and N types, for example, A/Fujian/411/2002 (H3N2), the type that seems to be most common this year. When the H or N types completely change, that is a “shift” and is more of a problem because we are less likely to have antibodies to viruses that have such a major change. Most influenza we have experienced in recent years is H3N2. The Spanish flu epidemic of 1918-19 was H1N1. The Asian flu epidemic of 1957-58 was H2N2. The Russian flu of 1977 was H1N1.

If you’d had enough of history and science, then let’s focus on the current flu season. If you have not received a flu shot yet, there is still time. Influenza is currently sporadic in Pennsylvania with limited activity in other Eastern states. As people return from holiday travels and come home from school for the holidays, influenza will be carried like an unwanted holiday gift from Western states where it is becoming widespread. Since flu vaccine is never 100 percent effective, the more people in an area who are immunized, the less likely anyone else will get it. The only people who should not get flu vaccine are those who have a life threatening reaction to eggs, children under 6 months (it does not seem to help them) and women in the first three months of pregnancy, since it can not be proven to be completely safe at that time. Flu vaccine does not cause the flu or make a person more susceptible to colds.

There are some specific treatments for influenza but they should be started early and while they may shorten the course of the flu, they do not reduce the chance of hospitalization. As usual, prevention is better than treatment. If you have never had the flu, it’s no guarantee you won’t get it this year. There is plenty of vaccine around, so get your shot soon.

For more information, visit http://www.cdc.gov/ncidod/diseases/flu/fluvirus.htm