A Little About What the Flu Vaccine Is and How It Works
Every year, public health experts urge everyone six months old and up to get a seasonal flu shot. They are especially eager for all of us to be vaccinated against the virus that causes flu this year because of concerns that the healthcare system would be overwhelmed this winter by flu cases because of the surge in COVID-19 cases.
Yet many people in past years have not gotten the flu shot (although the rates do seem higher this year) and some people think it is either unnecessary or causes the flu itself. Indeed, if you really start getting into the details about the flu and the vaccine against it you can run up against some complicated terminology and concepts. While the best place to get information about the flu shot is from the CDC, even their explanations for the general public can seem complicated. Given how important getting a flu shot is this year, we thought it might be wise to focus on it for the latest edition of our occasional series on vaccines.
First, What is the Flu?
What we call the flu is an illness caused by the influenza virus. This is an RNA virus. In humans, our DNA makes another molecule called RNA, which then makes the proteins that do the body’s work. The influenza virus has no DNA, just a single strand of RNA inside a capsule. Like all viruses, it does not have the machinery to reproduce on its own, so it must enter cells of organisms higher up the chain. When it does this, the virus’ RNA hijacks the invaded cells’ reproductive machinery, causing it to manufacture viral proteins and many new copies of the viral RNA. That RNA is then put back into the capsules and released into the blood stream so it can infect other cells.
Influenza virus becomes airborne when an infected person speaks, coughs or sneezes. Another person gets it either by breathing it in or touching a surface that has just been touched by someone with the flu. Hence, you can get infected by just shaking hands if you then immediately touch your face and the virus gains entry into your mouth or nose that way (which is why once again washing your hands right away if you do have contact with someone who has flu is a good idea). Most flu is transmitted from human to human, but some types of flu like bird or avian flu and swine or pig flu can be transmitted from animal to human by handling infected animals or eating undercooked food. For reasons that are not entirely clear, the flu is almost entirely seasonal and generally strikes only in colder months.
The symptoms of flu are well-known and usually include fever, cough, sneezing, running nose, sore throat, muscle and body aches, and headaches. It usually lasts just a few days and most people recover. But some people, especially those at high risk like children under five (and especially under 2), elderly people, and people with underlying medical conditions, can get serious complications from the flu, requiring hospitalization and even admission to an intensive care unit (ICU). The most common of these severe complications is pneumonia, which can be caused either by the influenza virus itself invading the lungs or by another virus or bacteria that takes advantage of the situation because the immune system is involved in fighting the flu and is therefore in a weakened state to defend against other pathogens. Besides pneumonia, other complications include inflammation of the heart, brain, and muscles.
Every year since 2010, between 12,000 and 61,000 people have died from the flu in the U.S. That’s why public health officials fight the uphill battle every year of trying to convince us that the flu is not always a mild illness that you just get over. People die from it, including small children.
The Complexities of Flu Vaccines
Given the potential severity of the flu, it is no wonder that public health and infectious disease experts urge us all, with just a few exceptions, to be vaccinated. Yet here is where complexities and misunderstandings have gotten in the way. In the last 10 years, fewer than half of the American population received an annual flu vaccine, with children getting it much more often than average and those 18-49 getting the flu shot less often.
There are very few people who should not get an annual flu vaccine—children under 6 months old and people who have had severe, life threatening allergic reactions to the vaccine or one of its components in the past are among them. So why do so many people shun the only known why to prevent the flu?
The first excuse is that the flu is not serious enough to require a vaccine. We’ve already pointed out how wrong that is, but because most people indeed do get over the flu without any complications it may seem like fearmongering to constantly warn about the severe cases and deaths from flu. That fortunate experience, however, does nothing to change the basic fact that some people, including those who have had mild cases in the past, do indeed get very sick with the flu and even die. Every year anywhere between 140,000 and nearly one million people are hospitalized with flu. Unlike COVID-19, flu can be a very serious illness in young children. We know that sometimes merely spouting large numbers like this is numbing and doesn’t really bring the seriousness of a situation home. So we need more stories of individuals who have gotten very sick from the flu in order to put real faces to the problem. If you send Critica such stories and give us permission, we will publish them on our website.
A second incorrect notion about the flu vaccine is that it can cause the flu. You can get soreness around the area where the shot was administered and fever, headache, muscle aches, and fatigue, from the flu vaccine, but these reactions are almost always mild and last a day or so. You cannot get the flu from the flu shot and here is why.
The influenza vaccine works by stimulating the production of antibodies against the virus that act to neutralize its effects. It takes about two weeks after receiving the vaccine for that to happen and that is why some people think they got the flu from the flu shot—they were probably either exposed just before getting the shot or during that two week period when the vaccine hasn’t yet become fully effective. In fact, the vaccine itself can be composed of one of three things, none of which can actually cause the flu.
Remember that the first thing the viral RNA does when it enters one of your cells in your nose or mouth is to recruit your own cell machinery to make viral proteins. These proteins are what are technically called antigens, agents recognized by the immune system as foreign and therefore arousing an immune response that includes making antibodies. So vaccines work by exposing you to enough of the viral antigens to cause the immune system to react and remember what those antigens look like, but not enough to cause disease. That can be done in the case of flu vaccine by giving either:
1. Inactivated flu vaccine composed of viral particles that have been killed. They still have the proteins on them so that an immune response can be triggered, but because the virus is dead they can’t cause any disease. These are usually made in chicken eggs.
2. Attenuated live flu vaccine containing viral particles that, although still alive, are from a strain that does not reproduce at body temperature. It is given as a nasal spray and survives in the colder environment of the nose but cannot infect cells in the rest of the body. It is not recommended for children under two, pregnant women, or people with a compromised immune system. These are also usually made in chicken eggs.
3. Recombinant flu vaccine, which involves only the proteins, or antigens, from the virus, which are manufactured inside mammalian cell cultures (not egg cells). These are packaged inside a different virus particle, one that has no ability to infect humans, and thus no flu virus is given at all, just its proteins. Again, this cannot cause the flu or any other illness in people.
Why Isn’t it 100% effective?
Perhaps the biggest misunderstanding about the flu vaccine is the concern that it doesn’t really work. After all, some insist, they always tell us it is not close to 100% effective and you have to have the shot every year. That does seem very different from so many other vaccines that provide years and sometimes a lifetime of protection. The measles vaccine is nearly 100% effective—almost no one who received the full course of MMR vaccines will ever get measles.
Once again, a little explanation about the flu virus itself may be helpful. The first thing to know is that there are many different types or strains of flu virus. Three main types—A, B, and C—cause disease in humans and within each of those categories there are several subtypes. Each of these subtypes of flu virus has different proteins and antigens. Most modern flu vaccines are either trivalent or quadrivalent, meaning they contain the antigens of three or four subtypes respectively. That may not be enough, however, so it is always possible that some disease-causing flu virus types emerge that are not covered by a given year’s vaccine.
To make matters even more complicated, flu viruses constantly change or mutate. That is, the RNA that controls the virus’ proteins changes itself and therefore slightly different proteins emerge. This process, called “antigenic drift” , occurs very rapidly in flu viruses, more rapidly than many other viruses such as the virus that causes COVID-19, which mutates much more slowly. The antigenic drift in the influenza virus occurs because of mutations that change the amino acid building block of proteins on the viral surface that are sites of antibody recognition. If sufficient antigenic drift occurs, antibodies stimulated by a flu shot in one year may not recognize the new viral strains. Every year, therefore, scientists must examine the virus to see what changes have occurred compared to the previous year. This allows updating of the vaccine and is the reason a new flu shot is needed every year. Even with this, it is still possible that unrecognized viral strains crop up not covered by a given year’s flu shot and therefore some people still get the flu even if they have been vaccinated.
Fortunately, this kind of rapid antigenic drift does not occur in all viruses and some vaccines, like the measles vaccine, can confer life-long protection for people who receive all the recommended shots. Scientists have struggled to develop a universal flu vaccine that would stimulate antibodies against parts of the influenza virus that doesn’t change so rapidly and therefore obviate the need for annual vaccinations. This is a promising line of research that should ultimately yield a more permanent flu vaccine.
There is no question that the flu vaccine is effective, just not 100% effective because of all the different viral strains and mutations. It is clear that the flu shot does not cause the flu, but it does substantially decrease the chances of getting infected with the influenza virus or getting very sick if one is infected. It isn’t a perfect vaccine, but it is far better than no vaccine.
By now, everyone who can should have gotten a flu shot. If you haven’t gotten it yet, please do. At the time of this writing in November, cases of COVID-19 are surging and the last thing we need is people with the flu requiring hospital beds to even further overwhelm our healthcare system. We also do not want to see people simultaneously develop both flu and COVID-19, because this might make someone even sicker than either infection alone. While we wait for a COVID-19 vaccine, please do not ignore the flu.