Covid vaccine immunity drops with time: who should worry more?

British doctors and the UK Health Safety Agency have warned that lowering immunity against Covid-19 could lead to death even in some people who have already had two doses of coronavirus vaccine in the country.

The drop in immunity is expected and should not be a reason for people not to get vaccinated, said in August, researcher Tim Spector, author of a study that found that the protection offered by Pfizer’s vaccine, for example, drops from 88% one month after the second dose to 74% after six months.

The Oxford/AstraZeneca vaccine, in turn, drops from 77% to 67% after the fifth month after full immunization.

With these falls, how far are we protected?

First, let’s emphasize the basics: Our immune system has two big roles, which are to keep us from becoming infected and, if that fails, to help our bodies get rid of the infection.

Now, imagine your immune system as if it were a medieval castle, surrounded by a vicious army of coronaviruses desperate to invade.

Your first line of defense is the outer wall, patrolled by a legion of archers. These are your body’s neutralizing antibodies. If they manage to fend off the viral army, you won’t be infected.

But if the wall and the archers are not enough, the virus will manage to enter the castle. You are now infected.

But beware: there are still troops inside the castle fortress. These are the immune memory cells, the B and T lymphocytes. Like knights, they can rally troops, lead the immune response, and drive out hostile invaders.

That’s because the Covid-19 vaccine has trained your body’s troops — which include both antibodies and these memory cells to react against an infection.

At least one of these lines of defense is diminishing, and it’s no surprise. This is what happens after any vaccine or infection.

“There is good evidence that antibodies decrease over time, which leaves us with obvious defects,” explains Professor Eleanor Riley, an immunologist at the University of Edinburgh, Scotland.

The desertion of these antibody archers from their posts was made worse by the emergence of the more contagious delta variant. It’s simply more efficient at getting into our bodies—it’s like a new army crowding into the wall with extra weaponry.

Perhaps you have realized the consequences of this for yourself: some people have taken Covid-19 even though they received the vaccines.

The drop in immunity over time is an obvious concern for governments trying to stop the advance of the coronavirus.

Adam Finn, a professor at the University of Bristol (England) and an adviser to the British government on vaccines, says the number of unvaccinated and vaccinated in UK hospitals has increased.

But he points out that “the protection that (the vaccinated person) has against mild infections decreases more quickly, whereas the protection against going to the hospital or against death decreases more slowly”.

The greatest risk of hospitalization or death is still concentrated in the elderly. The vast majority of deaths in people with two doses of vaccine occurred among those over 70 years of age.

It is worth remembering that vaccinated people in this age group proved to be much more resistant than the elderly who were unable or unwilling to be vaccinated. And, as is clear, the risk of younger age groups who have been double vaccinated remains small.

The passage of time ages all the cells present in our body—including those in the immune system. Aging makes it more difficult to build up immunity with vaccines, and it also makes the body’s reaction more difficult when the infection arrives.

“It is possible that the elderly had (good) protection initially, but now these antibodies have weakened and they may not have the second line of defense,” concludes Eleanor Riley. “This may be why we are seeing older, frail people dying despite having taken two doses.”

All this adds up to the fact that advancing age is sometimes accompanied by health problems. And, as the elderly public was among the first to be vaccinated against Covid, there was more time for this protection to diminish.

In the United Kingdom, as in Brazil, the elderly and other groups are already receiving booster doses against the coronavirus.

People with cancer, transplants, or other problems that damage the immune system have a slightly different problem in that their bodies don’t respond as well to vaccines.

“Their antibodies go down at a rate similar to that of healthy people, but they obviously start out in smaller numbers,” says Helen Parry of the University of Birmingham.

It is noteworthy that, between the Pfizer and AstraZeneca vaccines, there are important differences. “They seem to be good in different parts of the immune system,” adds Parry.

“The mRNA vaccines (in this case, Pfizer’s) are very potent at forming antibodies; AstraZeneca’s is very good at generating T-cell responses.”

Back to the castle metaphor, Pfizer’s vaccine seems more effective at protecting the outer walls with archers, while AstraZeneca’s is better at protecting the inner ones.

The good news is that, even with reduced immunity, the available coronavirus vaccines are exceptionally good. At the beginning of the pandemic, people dreamed of a vaccine that would reduce the death rate by 50%. Today, even with the natural effect of decreasing protection, the protection offered is still around 80% to 90%.

“Even the worst cases, six months later (from the start of vaccination in the UK), are better than we expected when we designed the vaccines,” says Finn. “They are still very good.”

Even better news is that there is evidence that the booster dose — already given to more than 11.4 million people in the UK and 10.2 million people in Brazil — is making an important difference. Data from the UK’s National Bureau of Statistics show that antibody levels rose in the older age group.

Now, the next step is to monitor to see if this additional dose will lower the number of cases and deaths.