How Long Will Immunity Against COVID-19 Last After the Vaccine?

(A crash course in immunity: An Entire Medical School Semester in 10,000 characters.)

One of the most common questions I am asked about the COVID vaccine is how long will the immunity against COVID last. The short answer is, we don’t know yet. Not only is that answer short, it is also short sided. Basic principles of immunity and some historical comparisons will shine some light on this issue. Also, immunity is multi-layered. It stretches far beyond the individual. In this post I am going to cover these topics to provide some better understanding of immunity and vaccines.

First, let’s dive into some basic principles of immunity. Your immune system, on the most basic level, is a system that detects and removes things that don’t belong inside you. The basic process is: detect the enemy, sequester the enemy, dispose of the enemy.

The detection phase occurs within hours of an infection. This is done by a variety of patrol cells that are constantly grabbing on to different things in your body to determine if it is you or something else. If they figure out its something else, they immediately destroy it and carry the pieces back to central bases in your body (lymph nodes, your spleen). This is your innate immune system. It’s the less evolved branch of your immune system.

Once there, they show the wreckage to the more evolved branch, your adaptive system. These are the B and T lymphocytes you've been hearing so much about. If they recognize the wreckage shown to them as foreign, the B and T cells begin the sequester phase. They confirm with one another to make sure the wreckage is foreign through a complicated system of checks and balances. Once confirmed the B-cell starts making antibodies. The T-cell then gives the recipe to all the other B cells to make antibodies. Pretty quickly your blood is flooded with them.

Antibodies are built specifically to grab onto the wreckage that was shown to the B cell. They are like these heavy plungers that bind to the enemy and keep it from moving and attaching to your cells. The handle part of the plunger sticks to other similar handles, so they all glob together in this big mess with the enemy attached to them. The enemy is sequestered.

Then final stage is disposal. This is done by these big garbage trucks called macrophages. They basically come in and swallow everything and break it down into harmless parts. Then they release signals for the cells around them to release a viscous fluid that you can excrete easily. You may have called this fluid, snot, or pus, or phlegm.

That’s the basics.

When you receive the COVID vaccine, your immune cells pick up the lipid capsule that houses the RNA for the spike protein. The RNA is released inside the immune cell. Your own cellular machinery starts making the spike protein until all of the RNA is used up. The spike protein is then displayed by your own immune cells to B and T lymphocytes. They recognize it as foreign and begin the process of building antibodies.

That’s how the COVID vaccine works.

Now we can start talking about how long immunity will last. The typical antibody will last within your blood stream for 6-9 months after an infection. Sometimes the time is longer. Other times it is shorter. Part of it depends on how severe the infection and how young you were when you were infected. You can imagine a severe infection will induce a significantly larger response than a mild infection. Your immune cells will make more antibodies and for a longer time in the setting of a severe infection. Infants and young children have immature immune systems that are learning how to fight disease. They tend to have very robust and swift responses to infections. Their immune systems will make very high amounts of antibodies and some of their white blood cells will continue to make antibodies for many years after the infection has passed.

Its ok that your antibodies go away. Your immune system has a cool memory function. When you are infected by something new your B cells will convert permanently into cells that will only make antibodies for that particular infection. Once they are assigned their infection, they go hang out in the bone marrow and wait until they see the infection again. They don’t need to be activated like in the process above. Once they detect the infection they immediately start dividing and make lots of antibodies.

When you are young you have a bunch of immune cells that are naive and untrained. They are willing to fight anything. When you are old, you have old staunch immune cells that only want to fight what they have seen before. You don’t have many of the undifferentiated naïve cells around volunteering to do whatever (kind of like real life). So, when you are old and get infected by something new, it’s hard to get that robust immune response you had when you were young.

This also explains why some immunity lasts longer than others. We can use historical examples to help guide us in understanding how long the vaccine for COVID-19 will last. For our historical examples we’ll discuss immunity to the influenza vaccine, and measles.

Warning- The influenza paragraph is a bit of a head-spinner.

Influenza is unique for several reasons. First, there are many versions of it. The virus has multiple origins (swine, bird, civet) and each origin of the influenzas virus is a different strain. There are two broad categories of influenza, A and B. Influenza A has two dominant surface antigens that vary from strain to strain, H and N. The strains are named based these surface proteins H1N1, H5N9 etc. Influenza B has four surface proteins that vary each year. Can you imagine the combinations? The vaccines are limited in the number of surface proteins that can be put in that little capsid, and the number your body can handle. You may have heard of quadrivalent (4) and trivalent (3). That’s it. I’m sure you’ve also heard someone say “I got the flu form the flu vaccine”. Basically, the vaccine really riles up your immune system.

Influenza mutates frequently as it travels around the world. It is commonly referred to as slippery, which means your antibodies kind of grab onto it and do their job, but it takes a while.

Everyone has heard something like, the flu vaccine was only 45% effective this year. As you can see, you have to pick the right antigens, avoid mutations, and deal with two different flu strains. Honestly, 45% isn’t that bad. Also, considering we only vaccinate between 40-50% of the population, it’s surprising we don’t have more issues.

This is why we have a flu season every year.

The good news is, if you have antibodies to a particular flu strain, they last a really long time. The H2N2 strain has not circulated since before 1960. A few years ago, clever researchers checked for antibodies against H2N2 in individuals that were alive during that time, and they were still there! Now, the flu vaccine has only been around since 1999. However, research has shown that the flu vaccine induces a similar response to natural infection.

The measles virus doesn’t mutate. Measles is measles, and will always be measles. Research shows that antibodies to measles from the measles vaccine can last into early adulthood and then they typically begin to fade (20 years). They can still often be detected in titers but not at a level that would infer immunity. However, the memory defense will remain. This is the genius of childhood vaccination. By vaccinating all the potential carriers that don’t have immunity, the virus, even though it is around, has nowhere to go because immunity lasts long enough, and the virus doesn’t change. An adult that decides not to vaccinate their children risks not only their children becoming infected, but also becoming a carrier. This is due to the lag-time for the immune response to build after exposure to the virus. This is mainly the mechanism for local outbreaks of measles. One child becomes infected, then asymptomatic carriers spread it to other adults and non-vaccinated children. In the setting of outbreaks, it is generally recommended that all potential exposures get revaccinated against measles.

In summary, the length of your immunity is dependent upon the immune response to the vaccine and the behavior of the virus.

You can anticipate that after you get the COVID-19 vaccine, your body will make lots of antibodies. Over a period of 6-9 months those antibodies will fade. Immature immune cells in your body will convert into permanent COVID-19 fighting cells and be ready to fight it if they see it again. These cells will likely live somewhere between 20-50 years waiting in your bone marrow. After your antibodies fade in 9 months you will be able to contract COVID again, but your case will likely be asymptomatic (carrier) or very mild.

If the virus behaves like influenza the vaccine will need to be tweaked and you may have to get it again.

If there continues to be many cases of COVID in the next 18-24 months you will likely be asked to get a booster of the vaccine because even though you are immune, you can still be a carrier once your antibodies are gone, which brings me to my last point.

When you think about immunity against a disease you have to think in multiple layers. There’s personal immunity, community immunity, and eradication.

If you get the COVID vaccine you are immune. Which is great. However, your community is not. If only 50% of eligible people get the vaccine then there are still going to be cases, and we will be in a seasonal flu scenario. In order to suppress the virus, you will have to keep getting rounds of the vaccine. If your entire community gets the vaccine, it becomes completely suppressed. As immunity wanes over decades there will be outbreaks and re-vaccination will be required. This would be very similar to the measles scenario I discussed above. That is community immunity. Then there is eradication, in which everyone gets the vaccine, the virus has nowhere to spread, and it dies in its final host. This was the fate of smallpox, which was a devastating disease that met its match against the very first vaccine.