There is growing evidence that people who have received COVID-19 vaccines are less likely to become infected with the virus and they are less likely to have symptoms of severe illness.
However, evidence is also emerging that existing vaccines may offer less protection against new variants of SARS-CoV-2, such as the delta variant.
Israel has already started to offer booster shots of the Pfizer vaccine to at-risk adults, and the United States is in talks with Pfizer about introducing these shots for the most vulnerable.
There has been talk of a third dose for some COVID-19 vaccines. Why might vaccine boosters be necessary, and what are the questions researchers are yet to answer about them?
All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.
One studyTrusted Source, for example, showed that 95% of people who received both doses of the Pfizer-BioNTech or Oxford-AstraZeneca COVID-19 vaccine had a weaker immune response to the delta variant than to previous strains.
People who received just one dose of either vaccine had a very weak immune response, which suggests that a single dose of one of these vaccines does not offer adequate protection.
The Health Ministry of Israel also released a statement saying that the effectiveness of the Pfizer-BioNTech vaccine in preventing symptomatic COVID-19 fell to just 64% at 6 months post-vaccination. It noted, however, that two doses of Pfizer are still able to prevent serious illness in 93% of cases.
As of yet, the U.S. has made no decision as to whether they will approve access to booster vaccines. However, the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) said in a joint statement that they are “prepared for booster doses if and when the science demonstrates that they are needed.”
What are vaccine boosters?
Vaccine boosters are additional vaccine doses that should provide extra protection against disease, as the effects of some vaccines may wear off over time.
Vaccine boosters are common for many viral infections, including the flu, which requires a booster every year, and tetanus, diphtheria, and pertussis (DTaP), for which a booster is necessary every 10 years.
For some vaccines, receiving smaller doses more often is more effective than getting a single large vaccine dose.
This approach allows the immune system to build its immune response sustainably. When the immune system remembers certain antigens that previously activated it, it can respond much quicker the next time it encounters them.
Although many vaccine boosters are identical to the previous doses, some are modified to enhance their efficacy. The flu vaccine, for example, changes every year to respond more effectively to new mutations of the influenza virus.
Why take booster doses?
There are generally two reasons why people might consider vaccine boosters. The first is that immunity naturally wanes over time. Without repeated exposure to certain antigens, the immune system may become less able to prevent infection or disease. Vaccine boosters help the immune system maintain a protective response.
“Another reason we may need booster vaccinations is viral variants,” a spokesperson from the COVID-19 Vaccine Team at the University of Oxford told Medical News Today. They went on to explain:
“Some variants have evolved to avoid some parts of our immune response, which means they can more easily infect those who have an existing immune response to the virus — i.e., those who have been previously infected or vaccinated. However, the virus cannot avoid all parts of our immune response. A booster vaccination is helpful, as it can improve the parts of our immune response the viral variant cannot avoid.”
“Alternatively, we can use a booster vaccine that specifically targets the viral variant. This works by producing a new immune response to the parts of the virus which have changed from the original vaccine whilst also improving the existing immune response against the unchanged parts of the virus, which also should help protect against other variants,” they added.
Who should consider a booster shot?
As national agencies such as the CDCTrusted Source note, existing data indicate that most COVID-19 vaccines produce a strong immune response that offers sufficient protection against the SARS-CoV-2 virus.
However, it remains unclear for how long COVID-19 vaccines — in the current dosages — continue to offer protection.
Nevertheless, booster doses might benefit older individuals or those with a weak immune system,as their bodies may not have generated a strong enough immune response after the initial vaccines.
“Based on current data, for people who respond well to the vaccine, it looks like immunity remains strong for over 12 months and works even against the new variants,” Dr. Richard Stanton, a reader in the Division of Infection and Immunity at Cardiff University in the United Kingdom, told MNT.
“My personal opinion is, therefore, that we don’t need to be too worried about boosting them yet,” he said.
Dr. Jessica Justman, associate professor of medicine in epidemiology at Columbia Mailman School of Public Health, further explained that “[b]ooster shots might be most beneficial for those who have medical conditions — such as some solid organ transplant recipients (e.g., kidney transplant) and some individuals with autoimmune diseases (e.g., lupus) — that prevent them from developing an effective immune response after the first round of vaccination.”
However, she added, “[w]e would first need evidence that a booster shot for these individuals is safe and effective.”
Other scientists say that the opportunity cost of not providing booster vaccines outweighs the lack of research underpinning their use.
In an interview with the BMJTrusted Source, Prof. Anthony Harnden of the Joint Committee on Vaccination and Immunisation in the U.K. expressed the opinion that the U.K. is likely to roll out booster vaccines and that annual boosters might be necessary for some time.
Controversies around vaccine boosters
“Scientifically, there is nothing controversial about additional booster vaccinations; they work in very much the same way as a second vaccine in a two-vaccine schedule,” the Oxford COVID-19 Vaccine Team spokesperson told MNT.
“The debate around additional vaccines is whether they are needed at this stage or [whether] vaccine doses are better off being used elsewhere — particularly in poorer countries, as highlighted recently by Sir Andrew Pollard in an article in The Times newspaper,” they added.
Sir Andrew Pollard, director of the Oxford Vaccine Group at the University of Oxford, wrote that the main purpose of vaccines is to “keep people out of hospital.”
As vaccines seem to reduce hospitalizationsTrusted Source with COVID-19 significantly, and experts predict that vaccine supplies will be limited for the foreseeable future, Sir Andrew argues that it is important to prioritize those who have not yet had even a single shot before providing boosters to others.
Some people even question whether it is morally right to give booster shots to those who have already been vaccinated when many people, especially those in developing countries, have not received even one shot and are thus at a higher risk of infection.
Source: Medical News Today