For those holding on to hope of an imminent Covid-19 vaccine, the news this weekend that the first could be rolled out as early as “just after Christmas” will have likely lifted the spirits. The UK’s deputy chief medical officer, Prof Jonathan Van-Tam, reportedly told MPs a vaccine developed by Oxford University and AstraZeneca could be ready for deployment in January, while Sir Jeremy Farrar, Sage scientific advisory group member and a director of the Wellcome Trust, has said at least one of a portfolio of UK vaccines could be ready by spring. Much has been said about how the world will return to normal when a vaccine is widely available. But that really won’t be true. It is important that we are realistic about what vaccines can and can’t do. Vaccines protect individuals against disease and hopefully also against infection, but no vaccine is 100% effective. To know what proportion of a community would be immune after a vaccination programme is a numbers game – we must multiply the proportion of a population vaccinated by how effective the vaccine is. The UK currently has among the highest national coverage of flu vaccine in the world, vaccinating around 75% of the over-65s against flu every year; most countries either do worse or have no vaccination programmes for older people. It is reasonable to expect that this level of coverage could be achieved for a Covid-19 vaccine in that age group in the UK. Therefore, if the Covid-19 vaccine is 75% effective – meaning that 75% of those vaccinated become immune – then we would actually only protect 56% of that target population (75% of 75%). This would not be enough to stop the virus circulating. Almost half of our highest risk group would remain susceptible, and we won’t know who they are. Relaxing social distancing rules when facing those risks seems a bit like Russian roulette. Now let’s look at people younger than 65 in medical risk groups. In a good year, the UK vaccinates 50% of them against flu. That means just over a third of them are going to be protected (50% of 75%). Just to make matters worse, regulators such as the US Food and Drug Administration and the European Medicines Agency have said that they would accept a 50% lower level for efficacy for candidate Covid-19 vaccines. If that efficacy level is fulfilled, we have to multiply coverage by 50% efficacy, not 75%, and suddenly it all gets more concerning. As well as protecting individuals, vaccines can protect communities, through the interruption of transmission. One of the best examples comes from the UK meningitis C vaccination campaign of the late 1990s. There was a 67% reduction in the number of cases in unvaccinated children and young people because they were being protected by their contacts who had been vaccinated and were no longer transmitting infection. If we want to see population protection from a Covid-19 vaccination, we are going to need high levels of protection (coverage x efficacy) across all ages – vaccinating not just the at-risk groups, as is being planned. To stop transmission, we must vaccinate anyone who can transmit infection. Anything less means that our goal is only individual protection and not the interruption of transmission. A recent announcement from the head of the UK vaccine taskforce, that the strategy will be targeted vaccination, makes it abundantly clear that the UK vaccine strategy at the moment is not to try to interrupt transmission, despite having hundreds of millions of Covid-19 vaccine doses on contract. With less than 10% of the population showing evidence of having been infected, targeted vaccination will not allow “life as previously usual” to return. Even if countries do decide to switch from a personal-protection policy to a transmission-interruption strategy, obstacles remain. Much will depend on the successful vaccination (probably with two doses) of people who have not previously seen themselves to be at elevated risk. The challenge will be persuading the young, for example, to be vaccinated, not for their own benefit, but for the benefit of others. Adherence to recommendations for any Covid-19 interventions – social distancing, lockdowns, home working, cancelled holidays or vaccinations – depend on trust. If politicians are telling us that the present impositions on our lives are only going to last until we have vaccines, then the reality is that a false hope is being promulgated. Vaccines are probably the most powerful public health intervention available to us. But unless their benefits are communicated with realism, confidence in all recommendations will be put at risk. While hope and optimism are much needed in these dark times, it is important to be transparent. We need to communicate the clear message that although targeted vaccination may offer some protection, it will not simply deliver “life as we used to know it”. • David Salisbury is a former director of immunisation at the Department of Health and associate fellow of Chatham House’s Global Health Programme