Does COVID-19 infection provide long-lasting immunity? How about COVID-19 vaccination? The answers to those questions are critical as society moves forward to control further spread of infection. Unfortunately, no one yet knows exactly how long COVID-19 immunity persists, partly because the disease is so new and partly because individuals’ immune responses differ based on factors scientists are still working to understand. Studies so far suggest COVID-19 immunity may last years if not decades.
When the human immune system encounters a virus, such as the SARS-CoV-2 virus that causes COVID-19, it generates antibodies that target the virus and tag it for destruction by the immune system. It takes time for the body to build up enough antibodies to mount a defense. Generally, within a few days the bloodstream is teaming with antibodies. These antibodies linger in the blood for a while, gradually tapering off over a period of weeks or months. Other immune cells, specifically T and B cells, act as memory cells and stand ready to help the body generate a broad immune response should it encounter the same germ again in the future.
Researchers have learned that COVID-19 antibodies are still present in the blood of most people approximately six months after infection. They have also found that T and B cells are present up to eight months after infection. T and B cells are the key to long-term immunity: T and B cells developed in response to other diseases can persist for years or decades. Data so far has virologists and immunologists thinking COVID-19 immunity has the potential to last years.
The persistence of immunity after COVID-19 may depend on the severity of disease and the overall health of the individual’s immune system.
One study of 77 people who recovered from mostly mild cases of COVID-19 revealed that antibody levels dropped significantly in the four months after infection, but then gradually slowed. Eleven months after infection, researchers could still detect COVID-19 antibodies. (However, no one yet knows what level of antibody is necessary to mount an effective defense against infection.)
Some researchers believe that many people who have had COVID-19 will make anti-COVID antibodies for decades, according to an article by Turner et al. published in the May 2021 issue of the journal Nature.
Scientists are still studying the persistence of B and T cells generated in response to COVID-19. However, they know that some people who were infected with SARS-CoV-1, the coronavirus that caused the 2002-2004 SARS outbreak, still have relevant B and T cells. Some survivors of the 1918 flu pandemic still had some immunity to that particular influenza virus nearly 90 years later.
COVID-19 immunity after infection is not absolute. The first case of COVID reinfection was reported on August 28, 2020. A May 2021 brief from the World Health Organization (WHO) states that “available scientific data suggests that in most people immune responses remain robust and protective against reinfection for at least 6-8 months after infection.”
People who recover from COVID-19 infection, or who test positive and remain asymptomatic, are still encouraged to get vaccinated. Studies show that vaccination after COVID-19 infection and recovery increases the level of protective antibodies compared to infection alone. In people who’ve had COVID-19, unvaccinated people are more likely than vaccinated people to get COVID-19 again.
COVID-19 vaccines first gained emergency approval in the United States in December 2020. Investigators are learning more every day about immunity after vaccination. The level of protective antibodies begins to decline a few months after vaccination with all three available COVID-19 vaccines: Pfizer-BioNTech, Moderna and Johnson & Johnson (J&J). This is likely one reason for the increase in COVID-19 breakthrough infections starting about four months after the vaccination program began. The COVID-19 Delta variant is another reason: It is different enough from the original pandemic virus that antibodies your body made against the vaccine do not recognize the Delta virus as well. This allows the Delta variant to multiply undetected for a period.
But, just because antibody levels wane with time doesn't mean you have no immunity. Some form of COVID-19 vaccine immunity may persist for years, based on the presence of long-lived, specialized immune cells (B cells) in people who've received the COVID-19 vaccine. (Documented by the same group who showed that people who have had COVID-19 may make anti-COVID antibodies for decades.) This so-called "cellular immunity" may not block infection or mild disease, but it reduces the risk of serious or complicated COVID-19 and death. In fact, the vaccines are very good at protecting people from severe COVID-19 and hospitalization.
Due to signs of waning immunity and the Delta variant, the U.S. Food and Drug Administration (FDA) has been reviewing the vast collection of data on the safety and effectiveness of booster shots. Additional doses of vaccine after initial immunization can further stimulate the immune system; they are a common method of maintaining immunity against an infectious disease. The FDA authorized a booster shot (third dose) of the Pfizer-BioNTech mRNA COVID-19 vaccine for people initially vaccinated with the Pfizer vaccine. An advisory panel also recommended the FDA authorize boosters of the Moderna and J&J vaccines, but, as of October 16, 2021, the final decision has not been released.
There is still more to learn about COVID-19 immunity duration after vaccination and infection. It is an active area of research and ongoing clinical trials.
