Decoded: Understanding the debate over coronavirus booster shots

With the especially contagious delta variant threatening efforts to end the pandemic, a growing number of wealthy countries are planning or considering administering booster shots of Covid-19 vaccines, at least to particularly vulnerable groups. Officials at the World Health Organization have characterised this course of action as unethical as long as poorer countries still lack supplies to cover significant portions of their populations with initial doses. They argue that the strategy could wind up prolonging the pandemic, as well.

What’s a booster shot?

The term traditionally has re­f­er­red to an additional dose of a vaccine given some time after the initial course of inoculation to bolster protection that may have started to wane. While many vaccines produce long-lasting immunity, the US Centers for Disease Control and Prevention recommends that ad­ults receive boosters of the tetanus vaccine every dec­a­de, for example. For Covid-19, a new disease, rese­archers are working out the optimal schedule and dos­a­ge for a variety of vaccines on the fly in the midst of a pandemic. The term booster is being used loosely to refer to additional shots given for a variety of reasons to people who have already received the prescribed course of a Covid vaccine.

What are the reasons?

There’s a small group of people with weakened immune systems, such as transplant recipients, who are likely to need an additional shot sooner rather than later. The extra shot isn’t a traditional booster, as these people likely never get an adequate response to an initial course of Covid vaccine. For the rest of the population, an additional shot (or shots) may prove to be helpful if immunity wanes over time, or alternatively, if new coronavirus variants emerge that evade vaccine protection. In the first scenario, giving another dose of the original vaccine may be sufficient. That’s mostly what is being considered for the immediate future. In the second scenario, shots customised against new variants may be needed.

Which countries have signed on to extra Covid shots?

Countries that have started or decided to offer them to especially vulnerable groups inc­l­ude Israel, Germany and Fra­n­ce. Offering them more broad­ly to people months after their last dose are Russia, Hungary and the United Arab Emirates. Some nations plan to give extra shots using a vaccine type that’s different from the one people got initially. This mix-and-match strategy is called a heterologous boost, and there’s some evidence it can provide an advantage over an additional dose of the same formulation.

How are decisions about boosters made?

Since there is no scientific consensus for when booster shots become necessary, deciding when to employ another dose is a judgement call by public health officials. Accordingly, countries may make different decisions. In the US, the Food and Drug Administration official in charge of vaccine regulation, Peter Marks, has said that the US doesn’t have a “predetermined minimum” for how much efficacy must fade before it authorises booster shots, and will look at the totality of the evidence before making decisions.