Vaccine boosters help maintain protection provided by initial vaccination; boosters have been used for routine vaccinations like tetanus and polio for decades
Vaccination against COVID-19 protects against severe disease and death. But this protection may wane over time, making it necessary for people to receive another dose of the vaccine (booster) to maintain protection. COVID-19 vaccines aren’t the only vaccines to require boosters. Boosters are also part of several well-established vaccinations, such as those against tetanus and polio, and are part of the routine childhood vaccination schedule in the U.S. Whether a booster shot is necessary depends on the duration of protection afforded by a vaccine, and this varies depending on the disease and the vaccine in question.
Incorrect: While the initial dose of a vaccine induces a protective immune response, the strength of this response tends to wane over time, necessitating the use of booster shots to maintain protection. The requirement for a booster doesn’t mean that the initial vaccination didn’t work.
As the number of COVID-19 cases rises again, countries fortunate enough to have vaccinated a large proportion of their adult population against COVID-19 are now debating whether an additional vaccine dose (booster) might be needed for those who have already been fully vaccinated. U.S. health officials announced that those who received their second mRNA vaccine eight months ago could receive boosters from 20 September 2021 onwards. Israel already started giving boosters to people over 60 year old at the end of July, while Germany will start offering boosters from September. By 24 August 2021, over 12 million booster doses had been administered worldwide.
Amidst this debate, Facebook users shared the claim that boosters are being recommended “by admitting the first two [shots] didn’t really work as they said it would” in August 2021. But this is inaccurate. The fact that a COVID-19 vaccine booster might be needed doesn’t mean that the vaccine doesn’t work. A booster shot refers to an additional dose of a vaccine used to maintain protection in those who had been vaccinated before.
Boosters can become necessary because the level of protection offered by prior immunization declines over time. In fact, boosters have been a standard procedure for other vaccines that have been in use for decades, such as the tetanus vaccine.
Booster shots enhance the immune system’s ability to fight a pathogen
First, let’s look at how booster shots work. When a person is first vaccinated, their immune system is primed to fight the pathogen. The number of immune cells in your body that produce antibodies against the pathogen surges. But the number of these immune cells, called B cells, decreases over time. What is left behind is a small pool of memory B and T cells. These cells patrol your body for infections by the pathogen you have been vaccinated against.
Boosters build on this immunity. Ali Ellebedy, an immunologist at Washington University School of Medicine in St. Louis, who spoke to the magazine Nature, explained that a booster shot sets in motion a range of processes in your body. B cells multiply after an additional dose of vaccine. Boosters thus stimulate the production of more antibodies against the pathogen. Later on, the number of B cells will drop once more, but the pool of memory B cells that is left behind is now larger, meaning that a future response to the pathogen will be faster and stronger.
In addition, as Ellebedy described, boosters kickstart a process called affinity maturation. In this process, B cells travel to the lymph nodes. There, a process of mutation and selection leads to the production of antibodies that bind more strongly to the pathogen in the event of a future infection .
Vaccination schedules for several vaccines routinely include booster shots, as the protection given by the initial shots wanes over time. For example, a booster for tetanus, diphtheria and pertussis should be given every 10 years. Booster doses are also needed for the polio vaccine: in the U.S., children receive three doses for their initial immunization until they are 18 months old, and a booster dose between 4 to 6 years of age. Frequently, boosters are identical to the original vaccine. However, in some cases—most notably, the seasonal flu vaccine—the booster shot is modified to increase protection against new variants.
Needing a booster shot doesn’t mean that the vaccine doesn’t work
As a previous review by Health Feedback pointed out, full vaccination protects against COVID-19 in several ways, including by lowering the risk of infection, reducing the risk of severe disease in case of infection, and reducing viral load.
In the U.S., the CDC also recommended booster shots for COVID-19 vaccines on 18 August 2021, and beginning 20 September 2021, people in the U.S. may receive booster shots, scheduled to be given eight months after the initial vaccination, pending approval by the U.S. Food and Drug Administration. The CDC issued its recommendation because data showed that protection from infection induced by COVID-19 vaccines may wane several months after the initial shots, and a booster shot is given “to maximize vaccine-induced protection and prolong its durability”. But the CDC emphasized that the initial doses of COVID-19 vaccines still significantly reduce the risk of severe disease, hospitalization, and death.
Needing a booster shot does not mean that the initial vaccine didn’t confer protection, as Shane Crotty, a virologist at the La Jolla Institute for Immunology, pointed out in an interview with Scientific American. Crotty cited a study published in Science on the immunity generated by the Moderna vaccine, saying that “it looks like the vaccine generates high-quality immune memory”. According to the study, antibody activity persists for six months after the second vaccine dose.
A preprint (a study that hasn’t yet been peer-reviewed) investigated a booster of the Moderna vaccine which was modified to increase immunity to a virus variant. This study showed that the levels of antibodies against the original strain of SARS-CoV-2 “remained above levels likely to be protective” six months after full vaccination, but titers of neutralizing antibodies against some variants, notably the Beta variant, were low at the six-month mark.
After the booster was given, the levels of antibodies against these variants rose to or surpassed the peak levels measured one week after the second shot. Similar results were described in a press release by Pfizer-BioNTech on its Phase 3 trial of booster doses. According to the press release issued by Pfizer on 25 August 2021, levels of antibodies against the original strain of SARS-CoV-2 “one month after booster dose were 3.3 times the titers one month after the second dose“.
In summary, full vaccination protects against COVID-19, but this protection may wane over time. Booster shots are intended to maximize and prolong the durability of protection, and are in fact standard for several other longstanding vaccines, including the vaccines against tetanus and polio.
- 1 – Neuberger (2002) Antibodies: a paradigm for the evolution of molecular recognition. Biochemical Society Transactions.
- 2 – Pegu et al. (2021) Durability of mRNA-1273 vaccine–induced antibodies against SARS-CoV-2 variants. Science.
- 3 – Wu et al. (2021) Preliminary Analysis of Safety and Immunogenicity of a SARS-CoV-2 Variant Vaccine Booster. medRxiv