Correlates of immunity
The ACIP left much of its schedule open Sept. 23 to discuss extra Pfizer doses and vote on how they should be used.
Pfizer had originally applied to the FDA for an amendment to its FDA approval, which would have given doctors a freer hand to prescribe third doses as they saw fit, in patients as young as 16.
But the FDA’s Vaccines and Related Biological Products Advisory Committee voted Sept. 17 against granting the amendment. The committee was particularly concerned about the lack of data in teens ages 16 and 17, who have the highest risk for a rare side effect that causes heart inflammation that requires hospital care.
Instead, they recommended — and the FDA agreed per their decision Sept. 22 — that third doses should be given to people at higher risk for severe breakthrough infections because of advanced age or because they work in an occupation that puts them at high risk for exposure.
The CDC panel heard important presentations on new science that is helping to identify the correlates of immunity.
The correlates of immunity are biomarkers that can be measured in blood that help doctors understand how protected a person may be against COVID-19. These markers of immunity are not yet known for the COVID-19 vaccines.
Emerging evidence shows that booster doses of the Pfizer vaccine cause front-line immune defenders — called binding antibodies — to roughly triple soon after a person gets the third shot.
Neutralizing antibodies also jump soon after two vaccine doses, but they fall over time, which is natural. The body doesn’t need these foot soldiers to be on guard all the time, so they go away.
The body retains its memory of how to make them, however, so they can quickly be marshaled again, if needed.
Early studies suggest that antibodies account for about two thirds of a person’s protection against COVID, while the longer-lasting T-cells and B-cells account for about one third.
After the antibody levels fall, it may take a few days to recreate this army. In the meantime, the virus can try to break in. This can cause symptoms, which can make a person feel terrible, but for the most part, vaccinated individuals don’t need hospital care and are nearly always protected from dying — even against the Delta variant.
Those most likely to be at risk for a breakthrough infection are older, because immune function wanes with age.
Essential workers, such as those who work in healthcare, may also benefit from high antibody levels, which can minimize symptoms and help them get back to work more quickly.
Helen Talbot, MD, MPH, an associate professor of medicine at Vanderbilt University Medical Center in Nashville, said that in her area staffing levels are critical right now.
“I’m actually sitting in one of the deepest red [states] with high rates of COVID. We don’t have enough health care workers currently to take care of the unvaccinated,” she said.
“When we have beds, we are often missing staff, and so the idea of vaccinating health care workers is to be a little bit different than our idea of using vaccines in the general population,” Dr. Talbot said.
Oliver Brooks, MD, chief medical officer of the Watts Healthcare Corporation in Los Angeles, said he was in favor of making a public statement about the temporary nature of the potential recommendations Sept. 23, because they probably won’t cover all who might need a third shot.
“We may want to go on record stating what it is that would allow us to broaden our recommendation or restrict our recommendation,” Dr. Brooks said.
The considerations of who should get an extra dose are not always straightforward.
New modeling by the Harvard TH Chan School of Public Health and the CDC to assist the government’s decisions on boosters had a surprise finding: in nursing homes, it’s more effective to vaccinate healthcare workers than it is to give booster doses to these residents. Nursing homes are at the mercy of community transmission.
In regions with high transmission, it’s easy for a caregiver to bring the virus into a facility — so the models found that the transmission from these workers is a more effective strategy than giving third doses to the already highly vaccinated group of seniors who live in them.
A version of this article first appeared on.