The Atlanta Journal-Constitution
What to know about COVID-19 booster shots
COVID-19 booster shots are being rolled out in the U.S. and other countries in response to waning antibody levels in already vaccinated individuals and the increased threat posed by the hyper-infectious delta variant of the coronavirus that causes the disease. Giving a third shot to healthy people is a contentious strategy, since many low- and middleincome countries have yet to immunize even a 10th of their population. From a scientific standpoint, though, there’s mounting evidence that it could help stem transmission and avert hospitalizations and deaths.
1. What’s happening to antibodies?
Antibodies are needed to neutralize or block the virus, called SARS-COV-2, before it can invade the cells lining the nose and throat, where it first replicates to cause an infection. Numerous studies have reported a reduction in antibody levels over the six months following full immunization. But the degree to which that occurs with COVID-19 shots appears to vary depending on factors including:
■ The vaccine type and the amount of active ingredient in it
■ Prior exposure to the virus, either from a natural infection or a previous shot
■ An individual’s immune response, which is influenced by age and preexisting diseases, especially those that weaken the immune system
■ The interval between doses
2. How significant are waning antibodies?
Scientists don’t really know the implications yet. One reason is that antibodies are only a part of the immune response — other components also play critical protective roles. For instance, vaccination generates a durable immune memory. So-called memory B cells mature and increase in number over six months, so that if another infection occurs, new antibodies can be made that are better at blocking coronavirus variants. Additionally, high levels of vaccine-induced T cells, a type of white blood cell capable of finding and killing virus-infected cells, can be detected after six months, helping to ward off serious illness. That means that, while a decline in antibody levels over time might result in an increased risk of breakthrough infections, vaccination remains highly effective at protecting against developing a life-threatening case of COVID-19.
3. Do booster shots help?
Preliminary findings from a study run by the U.S. National Institutes of Health and presented in October found that booster shots were well-tolerated and increased antibodies in adults tested — and that mixing the shots could provide an even greater benefit than receiving another dose of the same brand. That’s one of the reasons the U.S. Food and Drug Administration approved a so-called “mix and match” strategy for certain high-risk individuals. An earlier study done on a small group of patients on dialysis found a third dose of the so-called MRNA vaccine made by Pfizer and Biontech dramatically raised antibody levels, by a median of 580%, over those who only had two doses. Soren Brostrom, head of the Danish Health Authority, said other research that’s been submitted for regulatory review but not yet published indicates that a third dose of an MRNA vaccine given to healthy adults should trigger a rebound in antibodies to peak levels.
4. What would that do?
Higher antibody levels should help prevent the coronavirus from both infecting cells and causing illness, and reduce transmission. Delta virus particles are much less likely to be infectious if they’re emitted from fully vaccinated people, researchers in the Netherlands showed in a study released in August, ahead of publication. Immunized people are also infectious for a shorter period compared with those without immunity, reducing the likelihood of onward transmission, research from Singapore showed.
5. Are boosters working in the real world?
Early data from Israel, one of the first countries to vaccinate widely, is encouraging. It began offering third shots to vulnerable people in July and gradually expanded the program after breakthrough infections led to a surge in cases. One study indicated the boosters helped curb transmission and severe illness. A separate study found people who received a supplemental dose of the Pfizer-biontech vaccine had a 48% to 68% lower risk of infection a week to 13 days later, compared with those who got the standard twodose regimen. The protection increased with time, with a 70% to 84% reduced risk of testing positive two weeks to 20 days after getting a third shot. The U.S., U.K. and other countries are also pursuing a booster strategy, so more data should be forthcoming.
6. Why is delta so challenging?
It’s been shown to replicate faster inside the nasal cavity than was observed with previous strains. So an infected person has more virus to transmit to others — and one or two days sooner, potentially well before developing any symptoms. That makes contact tracing more difficult as well. Delta is also better at evading preexisting immunity, rendering vaccines less potent. The more the virus circulates, the more opportunity it has to undergo further genetic changes that could give rise to new variants that may be even more pernicious. Plus, not all breakthrough cases in vaccinated people are mild; a minority of people will come down with a severe illness. People could also develop persistent symptoms or conditions associated with so-called long COVID-19.
7. Why is it contentious to offer boosters?
Large swaths of people in many nations have yet to receive any coronavirus vaccine. It’s believed their access will be reduced further with the mass rollout of third doses in better-resourced countries. In some nations, wealthy individuals are obtaining an extra dose while most of their fellow citizens remain unvaccinated. Booster doses in the context of such inequity is like handing out extra life vests to people who already have them, while we’re leaving other people to drown, says Mike Ryan, executive director of the World Health Organization’s health emergencies program.
8. Are there other ways of improving protection?
Yes. New vaccines, including variant-targeting inoculations, are in development that may improve protection. Scientists are also studying whether combinations of vaccine types and longer intervals between doses may also yield more robust immune responses. The data so far is encouraging.