Some immunity to the virus decreases. Why you shouldn’t worry.

Six months after vaccination, some immunity to the COVID-19 virus appears to wane.

Inoculation remains an essential tool against illness and death, but new data suggests that a booster may be needed to keep the vaccine’s strong protection against infection, at least for some people.

But it is not known when these injections will be available in the United States. “We will need to strengthen the safety of nursing homes with preventive measures until a recall is approved,” according to Andy Slavitt, one of President Biden’s top COVID-19 advisers.

Two new studies – one multinational review by Pfizer and the other in Israel, representing the longest period of data available to us to date – show that vaccines remain over 90% effective in preventing serious illness.

But the protection against infection begins to wear off six months later. Pfizer’s study of the initial 35,000 participants in its clinical trial shows 85% protection against infection at this stage, up from a maximum of 96.2%. The Israeli study, which is more controversial because its data set is so small, estimates that Pfizer’s shot is only 39% effective after six months.

The most vulnerable were the elderly and immunocompromised patients in nursing homes, according to the studies. These are likely the first groups to be eligible for a booster, said Dr. Bob Wachter, professor and chair of the UC San Francisco Department of Medicine.

The Food and Drug Administration and the U.S. Centers for Disease Control and Prevention are not yet recommending a booster vaccine, citing lack of data. But over the weekend, Israeli health officials said they would start offering Pfizer vaccine boosters to people over 60.

It is not clear why there is such a big difference between the Pfizer and Israel studies. This may be because the two populations were not identical based on age or other variables. Perhaps the PCR tests that detected an infection had different sensitivities. The interval between doses varied and Israel’s vaccination campaign started earlier. And because the Israeli data is so small, it requires much better analysis, said virologist Shane Crotty of the Vaccine Discovery Division at the La Jolla Institute of Immunology.

The sobering news comes as the CDC released evidence of clustered “breakthrough” infections among vaccinated people and recommended masks for fully vaccinated Americans who live in areas with high transmission rates.

The vaccines are modeled on the original strain of the virus, and it’s unclear whether the new delta variant pushes the numbers in the Pfizer and Israel studies.

How much should healthy Americans worry?

“I am not convinced, so far, that this is a danger for us – that in the short term we will need a strengthening of the whole population”, said Dr Joel Ernst, professor of medicine and head of the division. of Experimental Medicine at UC San Francisco.

“There is evidence of decreased immunity, but it’s not a steep downward curve,” he said.

The durability of our immunity to different pathogens varies widely, Ernst said. This is why boosters are routine for many other vaccines. Every year we lose our protection against seasonal flu shots. Our immunity to the tetanus vaccine wears off in 10 years, often longer. With hepatitis B, three doses of the vaccine provide lifelong protection.

The reason we lose our protection is that our antibodies, the body’s first line of defense against intruders, don’t last forever. They are not supposed to do it.

“It’s not a problem or a problem with the vaccine. It’s the natural response of the immune system,” said Dr. Monica Gandhi, infectious disease expert at UCSF. “If we kept all the antibodies in our bloodstream from every infection we saw, our blood would be thick as a paste. “

Our so-called memory B cells, when confronted with the COVID-19 virus, can simply stimulate new antibodies. “They just kick in,” she said.

Certainly, the highly transmissible delta variant creates challenges for our antibodies – a phenomenon that may explain the so-called “breakthroughs”.

Because the delta is multiplying so quickly, “what we’re probably seeing is there’s a lot more virus around so you get a big virus inoculum,” Gandhi said. “You may experience mild symptoms as your immune system starts fighting it. “

And antibody levels don’t present the full picture, although they are the easiest to measure. Other parts of the immune system will likely offer continued protection against disease and death, even without boosters, Crotty said.

Crotty’s study of protective T cells – which, among many other tasks, kill cells infected with the virus – found little change six months after vaccination. (There is no comparable data on memory B cells yet.) Unlike antibodies, T cells and B cells usually remain present.

“This is a good sign that suggests that in general immune memory is likely to be maintained long after the vaccine is given,” Crotty said. “The antibodies will stop the virus at the front door, but if a virus gets in, you have these other lines of defense.”

Here’s the challenge: It’s still not clear what level of immune protection is sufficient. Such a number – called a “correlate of protection,” as measured by the number of antibodies or other immune cells – would tell us if it’s time for a booster.

This magic number has been harder to find than it looks. There would be two different ways of approaching this research. With one, you would observe and wait for “breakthrough” COVID cases among those vaccinated, and then measure their antibody levels to see if they have dropped. Or you can do a “challenge test,” deliberately exposing vaccinated people to COVID, and then see if their antibody levels correlate with disease risk.