Most people do not need a fourth Covid vaccination

All of this talk has raised a depressing possibility: will we all continue to have shots every few months? Fortunately, the answer seems to be no.

The evidence suggests that most people don’t need a fourth dose. Immunocompromised people and the elderly who are at risk of severe breakthrough infections will likely need another vaccination – and may need to receive it until a longer-lasting vaccine is available.

But for most people, the immunity conferred by the first round of vaccination and natural infection will be enough to protect us. How can we be sure? We know the vaccines produce antibodies against SARS-CoV-2, the virus that causes Covid-19, but those antibodies may be less effective against new variants like Omicron.

Even if these antibodies are boosted with an additional injection, their effectiveness wears off a few months later. Fortunately, the vaccines also generate what is known as cellular immunity – which lasts longer – and provide more sustained protection against serious diseases.

It has been shown that antibody-producing memory B cells – generated by the vaccines or as a result of previous infection – recognize the virus, including its variants. Although we don’t know how long these memory B cells last, survivors of the 1918 flu pandemic were able to produce antibodies from memory B cells when their blood was exposed to the same strain nine decades later. The vaccines also trigger the production of T cells.

While B cells serve as memory stores to produce antibodies when needed, T cells amplify the body’s response to a virus and help recruit cells to attack the pathogen directly.

Memory T cells generated by Covid-19 infection can last a lifetime, according to a study that looked at participants with varying degrees of severity of the initial illness.

Memory T cells generated in people who survived another coronavirus infection in 2003 were shown last at least 17 years.

During the winter surge of the Omicron variant, we saw a greater likelihood of reinfection compared to earlier variants, but not in severe disease in the general population. A study from Qatar showed that a history of natural infection protected against mild reinfection with earlier variants 90 percent of the time, but this protection against Omicron dropped to 56 percent.

Two vaccines back then provided 89 percent protection against mild reinfection against the Delta variant, but that protection dropped to 36 percent in a Canadian study against Omicron.

This is probably because the Omicron variant was able to evade antibodies. However, in both studies there was protection from severe disease, likely mediated by cellular immunity.

According to a study by the Centers for Disease Control and Prevention (CDC), the increase in antibodies from a vaccine boost can only last for a short period of about four months. But this boost will be necessary for certain groups to maintain protection against serious diseases, including the immunocompromised, the elderly and those with multiple risk factors.

This is because B cells typically take two to four days to make neutralizing antibodies, which can take too long to wait for those who are more susceptible to serious disease. Another recent CDC study found that those who were severely immunocompromised (eg, patients on B-cell-depleting therapies or recipients of solid organ transplants) and those over 75 with four comorbidities were susceptible to major breakthrough infections.

Immunocompromised individuals have already been approved for a fourth shot, and those who are elderly and have other medical conditions should obviously be next in line.

But there’s little evidence that a fourth shot would be beneficial to the rest of the US population.

A recent study in the New England Journal of Medicine showed that a fourth injection in healthcare workers did not improve vaccine efficacy or reduce viral loads.

In the future, We may need a booster shot that will expose us to the entire virus. Whole virus vaccines may provide better protection against multimutational variants.