As the first crop of people to be fully vaccinated are reaching the 6 month mark, the question is increasingly coming up regarding if and when we should get a third booster shot. There is some mixed messaging in the media, which is partly due to the fact that there is no definitive answer so far. But let’s review where we are.
There are two reasons why a booster shot might be helpful or even necessary to maintain immunity. The first is that immunity often wanes after vaccination. Immunity is not always fully maintained for a lifetime. The second is that as new variants arise they may not be as well covered by existing vaccines, and so boosters can be formulated to cover emerging variants. Do either of these factors require a booster to maintain adequate immunity, and if so for which vaccines? The short answer is – not yet, but we may be getting close.
Evidence for maintaining immunity so far is good. There are basic ways to measure immunity. One is to look for markers of immunity in the body, such as measuring circulating antibodies or B-cell function. The other (more direct way) is to measure the risk of contracting COVID-19. A study published in Nature in June 2021 found that B-cell markers of immunity persist at high levels in people who received either mRNA vaccine (Pfizer or Moderna) after 12 weeks, concluding:
Our studies demonstrate that SARS-CoV-2 mRNA-based vaccination of humans induces a persistent germinal centre B cell response, which enables the generation of robust humoral immunity.
A study published in The NEJM in June looked at antibody levels in people following the second dose of the Moderna vaccine. They found persistent neutralizing antibodies at 6 months. They also estimated the half-life of the antibodies by looking at the curves over time. Using two different methods they estimated the half-life of neutralizing antibodies to be 69 and 173 days.
Antibodies appear to persist for at least 6 months, and may remain for much longer beyond that, but how long does the actual protection last? In April Pfizer published data that indicates those fully vaccinated with the Pfizer vaccine had 91.3% efficacy after six months, with 100% protection from severe disease (as defined by the CDC). This data led the Pfizer CEO to announce that their vaccine is good for at least six months, but they cannot guarantee protection after that. That is not the same as saying it does not provide protection, only that we don’t yet have data beyond six months.
What about emerging variants, such as the delta variant? The good news is that the existing vaccines appear to cover this variant. They are especially effective at preventing serious disease resulting in hospitalization or death. However, there is some early data to indicate that they are not as effective.
For example, a study from the UK found that one dose of the Pfizer vaccine was only 34% effective against the delta variant, and 84% after two doses. This contrasts to 80% and 95% respectively for older variants. Data is similar with other variants. For example, this NEJM study finds that overall protection from the beta variant is 20% less than the protection indicated in the clinical trials, but protection from severe illness and hospitalization remained at 90%.
For the existing variants, therefore, total protection from infection is reduced, but protection from severe illness remains high. This is good news for those who are vaccinated, but suggests the current vaccines may be less able to prevent spread of the newer variants of COVID. This is concerning as new waves of infection are spreading around the globe.
What is the bottom line of all this? One lesson is obvious – get fully vaccinated. For the 2-dose mRNA vaccines, one dose is better than nothing but it is not enough. As new deadlier and more contagious variants are spreading, one thing is extremely clear from all the data, the vaccines protect very well against severe disease, including hospitalization and death.
Regarding booster shots, a recent joint CDC/FDA statement sums it up accurate. The evidence so far indicates that vaccine boosters are not necessary at this time. They are continuing to monitor the effectiveness of the vaccines going forward.
The WHO has a different perspective, although this is more politics than science. Two days ago the Director-General criticized plans by vaccine producers to produce booster shots.
“We’re making conscious choices right now not to protect those in most need,” Tedros said in Geneva.
He is concerned about wealthy countries topping off their COVID immunity while poorer nations have not yet received their first dose. While this is a valid point, the two priorities are not mutually exclusive. Developing new versions of the vaccines to cover emerging variants will help everyone, as it will help reduce the duration of this pandemic. We should do this while stepping up efforts to vaccinate the world, and when newer versions of the vaccines become available those updated vaccines should be the ones distributed. So again, everyone can potentially benefit from developing vaccines that cover emerging variants.
We also have to keep in mind that as newer variants, such as the delta variant, become dominant, we will eventually see variants of those variants. The virus will slowly mutate away from the existing vaccines, and we will have to keep up.
In terms of waning immunity, that needs to be followed, but the data we have so far indicates it should be good for at least a year. We obviously won’t know for sure until we get a year out from the first people to be fully vaccinated. So while scientists are not yet willing to say anything definitive, the expectation is that an annual booster shot is likely. This will extend high levels of immunity, and can cover new variants as they emerge. This will likely mean that we will all get our annual COVID shot along with our annual flu shot, which is very practical.
But official recommendations will have to wait until we get one year data.
