How Delta Is Bolstering the Case for Covid Boosters
(Bloomberg) -- Covid-19 booster shots are being rolled out in some 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 middle-income 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 of the pathogen.
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 shots appears to vary depending on factors including:
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?
One study done on a small group of patients on dialysis found a third dose of the so-called mRNA vaccine made by Pfizer Inc. and BioNTech SE 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 that a third dose of an mRNA vaccine given to healthy adults should trigger a rebound in antibodies to peak levels. Shane Crotty, a virologist and professor at the La Jolla Institute for Immunology’s Center for Infectious Disease and Vaccine Research in California, said they could be double or triple previous levels and also last longer.
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?
Time will tell, but 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 Inc.-BioNTech SE vaccine had a 48% to 68% lower risk of infection a week to 13 days later, compared to those who got the standard two-dose 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.K. and other countries are also pursuing a booster strategy, so more data should be forthcoming. Some health officials, including White House adviser Anthony Fauci, have said that immunization may require a three-dose regimen.
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 pre-existing 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.
7. Why is it contentious to offer boosters?
Large swaths of people in many nations are yet to receive any Covid vaccine. It’s believed their access will be reduced further with the mass roll out 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 are other ways of improving protection?
The Reference Shelf
- Related QuickTakes on the debate over booster shots, long Covid, the effectiveness of Covid vaccines, mixing vaccines, vaccine nationalism, mRNA vaccines, coronavirus variants, why delta is the most disruptive variant, breakthrough infections, and how these makes the goal of herd immunity harder to reach.
- Bloomberg Intelligence explains why delta’s bad attributes may warrant a new Covid vaccine.
- Journalist and author Zev Chafets, writing for Bloomberg Opinion, says Israel’s strategy of Covid coexistence is working.
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