As the world prepares for this year’s huge challenge of vaccinating billions of people, some may be wondering how often this vaccine will have to be made. Vaccine immunity for measles lasts a lifetime so you only need to take it once. On the other hand, immunity to the norovirus that causes stomach flu only lasts about six months, requiring frequent vaccination to inoculate a community from disease.

What about immunity for COVID-19? Since coronavirus vaccines have only been in use for a couple of months, it’s still unknown how long immunity lasts. Perhaps we’ll only need one vaccination our entire lifetimes. Perhaps we’ll need to get a vaccine every year like we do against seasonal flu. Perhaps it’s somewhere in the middle, requiring regular shots every couple of years.

“We simply don’t know what the rules are to inducing long-lasting immunity,” says Stanley Plotkin, a physician and emeritus professor at the University of Pennsylvania who began to research vaccines in 1957. “For years, we were making vaccines without a really deep knowledge of immunology. Everything of course depends on immunologic memory, and we have not systematically measured it.”

When scientists design a vaccine, they typically create a harmless version of the disease-causing viruses or bacteria in order to teach the immune system to recognize the real thing when the body comes into contact with a pathogen in the real world. The key to long-lasting protection seems to be memory B cells, immune cells that quickly expand and produce antibodies when confronted by a known enemy. These antibodies quickly latch onto the invading bodies, preventing infection. Vaccines can also train T cells, which can target infected cells when antibodies fail to do so.

Vaccine immunity may last longer than natural immunity following coronavirus infections

Many studies conducted early in the pandemic found that antibodies to SARS-CoV-2 wane after the first few months. This raised much concern that people may lose their natural immunity quickly, with the same likely happening for vaccine immunity. These doubts were amplified by research showing that immunity to four older coronaviruses —229E, OC43, NL63, and HKU1— that have been with humans much longer and are known to cause common colds lasts as little as six months to a year

But more recent studies have shown that reality is nowhere near as concerning. Research published in December 2020 by researchers at Monash University in Australia showed that people who have been sick with COVID-19 have immune memory to protect against reinfection for at least eight months.

Like other research before it, the study published in Science Immunology also found that antibodies against the virus started to drop off after 20 days after the infection. However, the scientists showed that memory B cells “remember” infection by the virus, and if challenged again, through re-exposure to the virus, they can trigger a protective immune response through rapid production of protective antibodies.

“These results are important because they show, definitively, that patients infected with the COVID-19 virus do in fact retain immunity against the virus and the disease,” Menno van Zelm, Associate Professor at Monash University Department of Immunology and Pathology and lead author of the study, said in a statement.

“This has been a black cloud hanging over the potential protection that could be provided by any COVID-19 vaccine and gives real hope that, once a vaccine or vaccines are developed, they will provide long-term protection.”

Another recent study published in December involving more than 12,000 healthcare workers in England — by far the most exposed group of people to the coronavirus on a daily basis — showed that those who were infected with SARS-CoV-2 continued to produce antibodies against the virus. According to the same study, just three people out of more than 1,400 antibody-positive individuals subsequently tested positive for SARS-CoV-2 from April to November 2020.

In other good news, a study published in The New England Journal of Medicine that investigated immunity in participants who received Moderna’s mRNA-1273 vaccine 119 days after the first vaccination found that they had more antibodies than people who recovered from COVID-19.

A lot of viruses, when they infect, also do things to turn off the mammalian immune response. So, it’s possible that a vaccine offers stronger, longer-lasting immunity against the coronavirus because it won’t have proteins that shut off the host immune response.

Some virus mutations could require new vaccines, but there’s no evidence so far of this happening

A person might be immune to coronavirus variants designed to fight current strains of SARS-CoV-2 but this immunity could prove ineffective against other mutated strains. That is the worst-case scenario and an absolute nightmare. The appearance of novel, more contagious coronavirus variants in the UK and South Africa have ignited such concerns.

Fortunately, there is no evidence that these new variants circumvent our currently designed vaccines, which are based on the spikes that sit atop the virus. Evidence also seems to indicate that COVID-19 isn’t as good at mutating itself as influenza – which means it could be unlikely we’d need an annual jab.

But even in the worst-case scenario, which is unlikely at this point, vaccine makers and other scientists can quickly adapt their vaccines. The main advantage of mRNA vaccines (Moderna and Pfizer-BioNTech) is that they can be tweaked extremely fast. Rather than inserting a weakened or dead virus, these vaccines insert only a small piece of genetic code that instructs cells to produce a certain protein or protein fragment. It’s very surgical work that can be performed quickly on a computer and by synthesizing chemicals. In literally a matter of days, scientists can have a new vaccine ready. Indeed, the cumbersome part would be repeating the clinical trials for safety and efficacy, which could take another six months to complete. But repeating these safety trials would only be required in special conditions, depending on how significant changes to the vaccine are.

Vaccination may still offer long-term partial protection

The more time passes since a vaccination, typically the less protected you are from the foreign invader the vaccine was designed to stop. The effectiveness of the flu vaccine is around 40% (compared to 90%-95% vaccine effectiveness for COVID-19), but this quickly drops to nearly 0% after about 150 days from vaccination.

However, in people who receive the flu vaccine but later become ill, the disease more often than not is markedly less severe. For smallpox, the vaccine prevents disease for a few decades, but is still powerful enough to shield people from severe illness than can kill.

Likewise, even though the immunity from the COVID-19 vaccine will wane, it will likely still offer potentially life-saving protection against severe disease. So, even if immunity against infection lasts only a couple of months, which is the case for the flu, the COVID-19 vaccine should still prove highly important in the long-run. After all, the problem with COVID-19 isn’t that people get infected in large numbers — the main problem is that too many people get seriously sick, overcrowding hospitals and resulting in fatalities. If a vaccine can tamp down COVID-19 and reduce its case mortality to that of the flu, that’s still a huge win for the world.

There are still many frustrating unknowns but the outlook is more optimistic than at the start of the pandemic

If you feel like this article doesn’t provide a straight answer, it’s because it really doesn’t — that’s simply where the science is at this moment. SARS-CoV-2 is still a young virus, known to the world for a little over a year. As such, we’ll have to wait for more research to answer some of the most pressing questions. Currently, all vaccine manufacturers are still monitoring study participants for long-term protection and safety for an additional two years after their second dose.

“Only time will tell – I suspect, but do not know, that we may need to get revaccinated periodically but I do not know at what intervals,” Hildegund Ertl, Professor at the Vaccine & Immunotherapy Center at the Wistar Institute, told ZME Science.

This article is part of a series by the ZME Science editorial staff meant to inform and educate the public concerning the coronavirus vaccines. This is not medical advice. You should consult your doctor before making a health decision based on information you read online.

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