CAUGHT A COLD? THERE ARE 98 REASONS WHY YOU’LL GET ANOTHER
Virtually everyone has experienced getting the flu and getting a cold. In general the flu gives us fevers and a profound weakness that lays us low for several days. With colds we fill our wastebaskets with tissue paper from our runny noses. Flu viruses have been studied for several decades and they lend themselves to vaccines. At my age, I usually take a shot each year and I haven’t had a flu since then (more than ten years). Why aren’t there vaccines for colds? Recently all known varieties of cold viruses in the world have been analyzed down to their nucleotide sequences.. They are called serotypes. There are at least 99 that fall into two categories of human rhinoviruses called HRV-A and HRV-B. Of the 99, 74 are HRV-A and 25 are HRV-B. As the name implies (the prefix rhino means nose) rhinoviruses cause runny noses. They are small viruses that make about a dozen proteins. They are spherical in shape and they can be found around the world.
Flu viruses tend to be far fewer in serotypes and fall into two categories that form the basis of which flu shot we get in the flu season which around here begins in the early fall. So far no one has figured out a way to make a vaccine specific for all 99 serotypes of HRV and that may be because each serotype requires its own vaccine. That means each of us is likely to have a cold every year (or even two or three colds in a given year). By the time we have hit our mid 60s we have had virtually all the serotypes available in the US and hence we rarely get colds when we get old. When I was younger I dreaded flying to meetings because I would tell my students that my cold germs would go into orgies with cognate cold germs they meet in another city or another country. After they swapped genes they came up with new forms of cold germs that caused me to come down with colds. I suspect if I went to Australia or to some parts of Latin America where I have not traveled, I would get a cold. In all likelihood I have experienced infections with the overwhelming majority of HRV cold viruses because I taught classes with hundreds of students every year (many of them seasoned travelers) and thus was more likely to be exposed to the available serotypes they brought to class.
Rhinoviruses are a major contributor to severe asthma attacks. They are RNA viruses and single stranded. They are related to polioviruses but differ considerably in their proteins, which have long followed independent paths to their favorite cell types for infection. Flu viruses are long folded viruses, like floppy noodles, that are quite different from rhinoviruses. In fact flu viruses are much closer to a form of virus that kills birds, called Newcastle disease. When a cold virus enters a human cell (usually in the nasal passageway), it introduces its RNA, which then multiplies rapidly and the HRV RNA produces a single large protein that is cut into 12 chunks. All 99 of the HRV viruses have an unusual clover-like sequence of RNA that allows them to rapidly synthesize their RNA in the host cell. The Wisconsin group that has worked out the 99 HRV viruses hopes they can locate a few vulnerable sites shared by clusters of these cold viruses so they can be prevented by vaccination. It may turn out a few years from now, that future generations will be at risk for colds only in their childhood years and the rest of their lives they will be protected by vaccination for those HRV serotypes with shared vulnerable sites.