First there was Alpha, then Beta, Gamma and Delta. Now, thanks to the tremendous efforts of scientists in sub-Saharan Africa, the world is getting to grips with the Omicron variant. This new variant of Covid-19 has a number of mutations that distinguish it from previous ones, raising concerns among scientists that the vaccines we are currently using may not remain as effective against it. In response, the UK’s Joint Committee on Vaccination and Immunisation (JCVI) has recommended that all over-18s in Britain should be able to get a Covid booster shot, as long as three months have elapsed since their last vaccine. The logic behind boosters is that ensuring the UK population has a high level of immunity will reduce the number of people requiring hospitalisation or dying as a result of the spread of Omicron. This is based on the hope that it is unlikely the vaccines we are currently using will be totally ineffective against the new variant, but it is too early to be definitive about this. So how do boosters work? Data from observational studies has suggested that the immunity built up by exposure to both vaccines and the virus reduces over time. Repeated exposure to viral proteins (contained within the vaccine, or encoded by it in the case of mRNA vaccines) leads to a more effective immune response when someone does come into contact with the Sars-CoV-2 virus – reducing the severity of the illness they experience and the chances of them transmitting the virus to someone else. A recent UK clinical trial, CoV-Boost, investigated whether people’s immune responses were improved when they had the same vaccine both for their first two jabs and their booster, or had a different vaccine for their booster. The trial studied groups of people who had received two doses of either the Pfizer or AstraZeneca vaccines, and randomised them to receive a third dose of one of four different Covid vaccines, with some receiving half-doses of these vaccines as their boosters. All the vaccines boosted antibody levels when given after two doses of the AstraZeneca vaccine, and six out of seven increased antibody levels when given after the Pfizer mRNA vaccine. But predicting the long-term protection provided by a vaccine is not straightforward, particularly when you take into consideration new variants. We often measure the level of antibodies circulating in the blood as a surrogate marker of immunity, but this tells us little about the responses of immune cells such as T-cells, which we know are important in fighting viral infections such as Sars-CoV-2. Knowing the level of antibodies also does not provide us with any information about the effectiveness of a vaccine at preventing severe disease that results in hospitalisation or death. The most important question for most people is whether they will get sick or die from Covid-19. Further data from the CoV-Boost trial collected over the 12 months after the booster vaccinations were administered will help answer these questions in the future. We don’t yet know for certain if the Omicron variant and other future variants of concern will result in us needing to get additional boosters. But it’s possible, for several reasons. First, it may turn out that our immune systems need regular exposure to Sars-CoV-2 viral proteins to remind them to generate an effective immune response. This will only become clear over time. Second, there is the risk of Sars-CoV-2 variants emerging that are so different to those currently circulating that we will need new vaccines entirely. Vaccine manufacturers are confident they can produce new vaccines even more rapidly than they produced the first generation. Other innovative approaches are being proposed by organisations such as the Coalition for Epidemic Preparedness Innovations, which has launched a funding call to encourage the development of Covid-19 vaccines that provide protection against multiple types of coronaviruses, avoiding the need for a new vaccine for every new variant that emerges. But it’s impossible to talk about Covid booster vaccinations without highlighting the importance of vaccine equity. Many people, including healthcare workers and people at risk of severe disease, have yet to receive any doses of a Covid vaccine, let alone a booster dose. To prevent the emergence of other variants of concern like Omicron, we need to drive down viral transmission everywhere. Otherwise we will eventually be dealing with the emergence of variants that are not substantially affected by the vaccines we have available, and the ground we have gained over the past year will be lost, along with many lives and the wellbeing of thousands upon thousands of people. You can think of the global pandemic like a house fire. Sars-CoV infections are raging at high levels in multiple territories across the world. Many high-income countries, including the UK, are attempting to tackle this fire by ensuring there is enough water available to douse the flames in the room they occupy. As long as infections are allowed to rage elsewhere because of a lack of global vaccines, the tinder for the fire will always be there. Simply rolling out boosters in one country, without delivering vaccines in low- and middle-income countries, is not going to prevent the house from burning down, or keep us safe from harm. Dr Charlotte Summers is a professor of intensive care medicine at the University of Cambridge