As more states expand eligibility and vaccination rates continue to rise, pre-pandemic normalcy is on the horizon. The question now on many people’s minds is how long vaccines will provide protection. While the short answer is it’s too soon to know with certainty, experience with other infectious diseases can help scientists make an educated guess. And it is good news – your immune system will probably be able to detect and quickly respond to the coronavirus for many years.
“Long-term data from vaccinated individuals will give us the definitive answer, but all signs point to the fact that a level of protection from vaccines should last for a significant period of time, potentially two to three years or longer,” said David Topham, Ph.D., the director of the University of Rochester Medical Center Translational Immunology and Infectious Diseases Institute. Topham and his colleagues at URMC have decades of experience studying the immune response to respiratory infections. His work on this subject has been featured recently in The Washington Post and The Wall Street Journal.
One of the sources of uncertainty is the simple fact that currently approved COVID vaccines are new, so researchers don’t have hard data on the durability of immunity to the virus. While around 100 million Americans have received at least one dose, the bulk of these vaccinations have only occurred in the last couple of months. However, data from a smaller group who participated in clinical trials provide some evidence that vaccines are producing a long-lasting immune response. Both Moderna and Pfizer-BioNTech recently announced that their vaccines remain 91% effective in preventing infection, and 100% effective in preventing severe disease six months after the second dose.
Antibodies are important, but not the whole story
When evaluating immunity, researchers often turn to antibody levels, or titers. Antibodies are created in the days and weeks after vaccination or natural infection. These cells bind with the COVID spike protein and prevent the virus from infecting cells and replicating. Once produced in sufficient quantities, these cells serve as a first line of defense, quickly detecting and snuffing out re-infection. However, antibody levels wane over time.
Two other cells in the immune system – memory B-cells and T-cells – hold the key to long-term immunity. As their names suggest, these cells are programmed to recognize specific infections and ramp up antibody production again. Unlike antibodies, memory B-cells and T-cells hang around for a long time.
“Memory T-cells and B-cells can stick with us for a lifetime,” said Topham. “While these cells cannot by themselves provide complete protection from re-infection, they can give the body a head start, which could mean the difference between severe and mild illness.”
Other coronaviruses provide us with further clues about immune durability. SARS, a coronavirus which first appeared in Asia in 2003 and spread to more than two dozen countries before it was contained, is similar to COVID. No vaccine was ever developed for SARS, but studies involving people who were naturally infected by the virus show robust antibody levels two to three years later. A study published last year showed that people who recovered from SARS retained memory T-cells immunity 17 years after infection.
Furthermore, two of the four circulating seasonal coronaviruses – source of the common cold – are in the same family as COVID. We are first exposed to these coronaviruses at a young age and repeatedly throughout our lives, and research suggests that the immune response lasts for years – at least in preventing severe illness. Immune cells from these coronaviruses may even provide a degree of protection from COVID.
What about vaccination vs. natural infection?
Natural infection also provokes a lasting immune response, but Topham believes the degree of protection could depend upon how sick you got. Research in patients shows that severe illness is correlated with higher antibody titers, with levels similar to those found in vaccinated individuals. However, natural infection immunity has risks, as seen in the phenomenon commonly referred to as “long haulers,” or people who have recovered from infection but are plagued with long-term health issues.
“The current COVID vaccines provide a robust immune response, which is why some people feel under the weather, especially after the second shot,” said Topham. “We see the same in people who recovered from severe illness. However, people who had mild or asymptomatic cases in all likelihood do not get the same boost to the immune system. The bottom line is if you were infected, you should still get a vaccine.”
What impact will variants have on immunity?
The good news is that data from currently approved vaccines show they appear to be effective in terms of preventing severe disease for most of the variants that have recently emerged.
“All viruses mutate over time and coronavirus is no exception,” said Topham. “Those mutations do not necessarily make them less vulnerable to vaccine-produced antibodies. It is important to remember that existing vaccines are based off the genetic sequence of the original virus that appeared in China, which is no longer in circulation. And yet the current vaccines remain highly effective.”
Mutations that occur in the genetic code of the spike protein are of particular concern. The spike protein actually consists of several different proteins, which provides antibodies many potential targets. Some mutations in the spike protein make it trickier, but not impossible for antibodies to do their job. That having been said, Topham speculates that variants could eventually emerge that evade the immunity provided by vaccines. “At some point it is reasonable to assume that mutations could occur in key areas of the spike protein and significantly diminish the protective properties of vaccine-produced antibodies.”
The mutation that scientists have their eye on is the South African variant, which appears to make the virus more transmissible and less susceptible to the antibodies generated by vaccines. Some vaccines have reported only 50% efficacy against infection from the South African variant.
Will I need a booster shot at some point?
The reality is that we are probably stuck with COVID for a while, at least until a significant portion of the world’s population is vaccinated, a milestone that may not be reached until 2022 or later. During this time, COVID could evolve to become a seasonal, mutating virus.
Because current vaccines provide a base level of protection which could last for years, Topham thinks it more likely that booster vaccines will be developed to target specific variants, in a manner similar to annual flu shots. Moderna, Pfizer-BioNTech, and others are already studying variant boosters and researchers will soon begin to study whether vaccines can be mixed and matched, so you may eventually get a booster from a different manufacturer than your original shot.