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With the highly transmissible versions of Omicron now causing nearly all of the COVID-19 infections in the U.S., most people have likely gotten infected, even if they’ve been vaccinated and boosted. So people are naturally asking whether they really need to get the latest booster shot, which is the first to target the Omicron variant. Most people rightly assume that after recovery, they’ve built up a pretty good immunity to the virus.
While that’s true, researchers are learning more about the different types of immunity that natural infection with the virus provides, compared to that afforded by the vaccines and boosters. Studies show that after natural infection with a particular strain of SARS-CoV-2, people do tend to develop significant levels of virus-fighting antibodies against that version of the virus. That response may even, in some cases, be broad enough to provide protection against a wider range of different strains of the virus. If you are infected, the immune system responds to all of the different proteins that the virus makes. When you’re vaccinated, on the other hand, the body only responds to the viral targets that the vaccines target, which is a more limited set of viral genes. That’s why the original vaccine, which contained genetic information from the first widely circulating strain of SARS-CoV-2, no longer appears to protect people from getting infected with the latest variants of the virus, specifically Omicron BA.4/5.
But there is a caveat to that potential advantage of natural infection. The immune response that the body generates is also correlated to the amount of virus the body sees. So if someone is exposed to and infected with a large dose of the virus, the body will produce a stronger, wider-ranging response than if someone is infected with a smaller amount of SARS-CoV-2. That means that not all natural infections are created equal, and there is generally no way for people to know how much exposure they have had once they’ve been infected. That information comes from lab-based PCR tests, which measure viral load, and most people aren’t going to doctors’ offices, clinics, or hospitals for COVID-19 testing anymore, instead self-testing at home with rapid antigen tests, which aren’t designed to provide information on viral load.
A 2021 study even found that not all people who get infected necessarily develop virus-fighting antibodies; in that trial, conducted with volunteers recruited at the University of Alabama at Birmingham, about a third of people did not develop detectable levels of antibodies even after testing positive on PCR tests for COVID-19. The researchers found that the more severe the symptoms people experienced, the more likely they were to produce antibodies, and many people infected with the recent Omicron variants experience mild or no symptoms at all, meaning that these people who were asymptomatic may not have generated appreciable levels of antibodies.
There’s also the question of how long protection from natural infection lasts. Regardless of whether you’ve been naturally infected or vaccinated, studies are showing that antibody levels, which are the first line of defense in protecting against infection, wane after several months. It’s also possible that vaccines produce a deeper type of immunity that involves not only antibodies but another type of immune cell called T cells that can remember and mount aggressive responses to a virus it recognizes if people get infected again. A 2021 study found that people who had COVID-19 and remained unvaccinated had two times higher risk of getting reinfected than people who got vaccinated after having COVID-19.
The bottom line is that any immunity, whether from natural infection or vaccines, doesn’t last forever. And because COVID-19 is a relatively new disease, researchers are still trying to tease apart how the body responds to the virus and what types of immunity the immune system generates. As that data grows, the most reasonable strategy at this point is to keep boosting immunity in order to gain the most protection possible from both getting infected with SARS-CoV-2 or becoming severely ill. That means getting booster doses even if you’ve been infected, about three months after you recover.