H1N1 Revisited – Why were the elderly less affected?
Note: for purposes of this post, I’ll be referring to people aged 65+ as “the elderly.” Apologies to older members of the population for the flinch-inducing phrase. It’s shorter than “the older members of the population” and nicer than “those blue hairs.” Moving on.
During a typical flu season both the youngest and oldest members of the population are most at risk for serious complications from influenza infection. However, during the 2009 N1H1 influenza pandemic, children and young adults under 25 were most likely to have serious complications from the virus, while those 65 and older did not seem to be as greatly affected as researchers would have predicted. Since H1N1 vaccination rates were low across the entire population, and the vaccine was not even available until part way through the spread of the pandemic, we know that older people were not more likely to have gotten the special 2009 H1N1 flu shot. So that is not a possible reason for the apparent immunity in the elderly. This suggests that older people were somehow already immune to the 2009 H1N1 virus before it appeared in pandemic form in Mexico.
A new study looking into this phenomenon appears in today’s issue of the journal Science. The study uses structural evidence from the N1H1 and related viruses to support the claim that many in the elderly population do have some pre-existing immunity to the 2009 H1N1 pandemic virus.
First a little about immunology, if you need it. If not, skip this bit. Or read it anyway, check my explanation. It’s all good.
So, we all know that there are specialized blood cells in our bodies that help us fight disease. We tend to lump all of these special cells together and call them white blood cells. There is a certain subset of white blood cells called lymphocytes. Lymphocytes are special because they can adapt to different diseases, learning to recognize a particular foreign invader, such as a certain type of bacteria or virus. A group of lymphocytes called B cells make antibodies, special Y-shaped proteins that stick to the invaders and help the other white blood cells recognize them as foreign things to be destroyed. If you have a healthy immune system, you will make antibodies for every foreign invader that enters your body, and these antibodies (and some of the B cells that make them) will stick around for the rest of your life to protect you against ever getting sick from that disease again.
This is why most people don’t get chicken pox twice, the antibodies they made the first time around help the body fight off the infection faster the second time the virus shows up, keeping them from getting as sick as they did the first time or maybe not at all. This is also how vaccines work. A vaccination gives you whole dead or weakened (attenuated) virus and the B cells make antibodies to that. Since it’s an immune response, people sometimes experience “flu-like symptoms” (aches, pains, fever, localized swelling) after a shot. This isn’t fun, but it is a sign that your body is behaving correctly and responding to the dead/attenuated virus. It’s also leaving a memory of that response in the form of antibodies, just like it would if you had been infected with live virus. Only without the nasty viruses-killing-your-regular-body-cells-and-actually-making-you-sick part.
Having antibodies to the foreign invader that causes a disease is called immunity. And believe me, immunity rocks.
So why do we get sick with colds and flu (and have to get flu shots) over and over? Because the new illness is not from the same virus that we saw before. You won’t get sick from an influenza virus you’ve been infected with before, or one that is similar to one you have been infected with before. If you have a normal immune system, you are immune to every bacterium and virus you have ever been infected with, whether it made you noticeably sick or not. Unfortunately, there are many different influenza viruses out there, and more evolving all the time, so chances are good you will become infected with a virus that you have not had before and get sick. The makers of the flu vaccine try to guess which 3-5 viruses out of the hundreds of viruses currently in circulation are most likely to make people sick in any given year and put those viruses into the vaccine. Sometimes they’re right and flu shots protect most people, sometimes they’re wrong and the flu shots seem to do no good.
Back to the paper
The authors of this study looked at the physical structure of the 2009 H1N1 influenza virus and compared it to the structures of other known H1N1 influenza viruses. They show that the 2009 virus is structurally similar to H1N1 viruses that commonly infected humans before the 1950’s, including, but not limited to, the 1918 H1N1 that caused the Spanish flu pandemic. They saw that the structural similarity is particularly good in areas of the virus where antibodies would attach to the virus and flag it as a foreign invader to be destroyed by the immune system. So the authors concluded that people who were infected with an H1N1 influenza virus in the early part of the 20th century are likely to be immune to the 2009 H1N1 because it looks like an influenza virus they have been infected with before. The antibodies they made while they were infected with the other, similar H1N1 in their youth stuck around and helped them fight off 2009 H1N1.
It was known before this study that sometime during the middle of the 20th century, the H1N1 type influenza viruses became less common in people and the H2N2 and H3N2 viruses became more common. So people born after the middle of the 20th century were less likely to have been infected with an H1N1 virus similar to the 2009 H1N1 pandemic virus, and so were less likely to be immune and more likely to get sick. The authors also note that various other H1N1 influenza viruses different from the 2009 H1N1 are also infecting people in the 21st century (as part of the “seasonal flu” virus bonanza). They show that these seasonal H1N1 viruses have antibody binding sites that are structurally distinct from the 2009 H1N1 pandemic virus, and so that is most likely why seasonal flu shots or normal immunity to the H1N1 seasonal viruses did not help younger people fight off the 2009 H1N1 pandemic virus.
The study was done by a group lead by Dr. Ian Wilson at the Scripps Research Institute in La Jolla, California, USA.