Despite the fine print, we can still feel optimistic about a Covid vaccine We could all do with some good news right now. After a long and difficult year of multiple lockdowns and numerous sacrifices, the announcement from Pfizer and BioNTech that preliminary results show their vaccine is 90% effective at protecting people from Covid-19 felt like the light at the end of the tunnel. The announcement is based on interim results from phase 3 of a multinational, randomised controlled trial Š considered the gold standard of clinical trials Š involving 43,538 people. Each participant was given two doses of the vaccine or a placebo 21 days apart, to see if this would prevent them developing Covid-19. To date, 94 of those enrolled in the trial have developed confirmed Covid-19. The data suggests that just eight of these had been given the vaccine. So far, there have been no serious safety concerns arising from the trial, which will continue until 164 participants have developed Covid-19. So what does this mean for the UK? When updating the nation on the vaccine on Monday evening, Boris Johnson cautiously stressed that these were still Ņvery, very early daysÓ. The clinical trial needs to finish and collect more safety data before the Medicines and Healthcare Products Regulatory Agency (MHRA) can even begin to review whether itÕs safe enough to grant temporary authorisation, allowing the vaccine to be rolled out before a full product licence is issued. But we should allow ourselves some cautious optimism. These preliminary results are likely a huge step forward. And this seems like a much better first-generation vaccine than many had anticipated. Not only are mRNA vaccines such as this a new technology, but 90% is a great starting point (even the World Health Organization has said a vaccine that was 50% effective at protecting people from Covid-19 would be acceptable). Of course, there are still hurdles to jump. Although we know the vaccine is 90% effective at preventing the disease, Covid-19, we donÕt yet know if it prevents infection with the Sars-CoV-2 virus that causes Covid-19. This is a crucial distinction: preventing infection from the virus, even in asymptomatic people who donÕt go on to develop Covid-19, may be as important as stopping people from developing Covid-19 and its attendant symptoms, because people can infect others with Sars-CoV-2 even when they have no symptoms. We also donÕt know if the small number of participants who became infected with Covid after receiving the vaccine developed severe or mild symptoms, which would indicate whether the vaccine can reduce the severity of Covid-19 and therefore limit the number of people who might need critical care or hospitalisation. And even when we do have the vaccine, or another like it, there will likely be challenges in rolling it out. UK guidance was issued in September about which groups would be the first to receive the Covid-19 vaccine. Older people, who are at greater risk from serious disease and death, will be a high priority, as will frontline health and social care workers, particularly those working in care homes. Matt Hancock has likened this process to boarding an aircraft Š with those in the highest priority group boarding the plane first. Although the clinical trials can tell us how effective the vaccine will be at protecting an individual from the disease, we wonÕt know how effective the vaccine will be at modifying the course of the epidemic across the population, and controlling the spread of the virus, until it has been rolled out. We will also need to continue seeking effective therapies for those with Covid-19, even when a vaccine is available. While the Oxford/AstraZeneca vaccine, which is currently in phase 3 clinical trials, has a manufacturing base in the UK that is already scaling up production, the Pfizer vaccine will be manufactured in Belgium. Pfizer predicts it will be able to supply 50m doses of the vaccine in 2020 and 1.3bn doses in 2021. This will not be enough to meet the global need Š meaning the countries that can afford to pay will likely be first in line, unless equitable access agreements are put in place. Related: If you're pinning your hopes on a Covid vaccine, here's a dose of realism | David Salisbury The vaccine has to be kept at an ultra-low temperature of -70C. Although the cold-chain delivery routes that allow for the cold storage of goods in transit arenÕt likely to be a problem in Europe, many other parts of the world lack the cooling facilities that would allow them to safely transport and store the Pfizer vaccine. This matters if weÕre going to suppress the virus globally, rather than only in high-income countries with well-developed logistics. But even if this particular vaccine canÕt be made in a form suitable for this, others will almost certainly be able to overcome these hurdles. This news has demonstrated the power of science to deliver solutions to this crisis. WhatÕs equally inspiring is the fact 43,538 people volunteered to participate in this clinical trial. Scientists urgently need more volunteers to do the same and there are many ways in which we can all help. If youÕve recovered from Covid, you can donate your plasma to NHS Blood and Transplant so the Recovery and Remap-Cap trials can determine if it is an effective treatment for Covid-19. Meanwhile, the GenoMICC study into why some Covid patients require intensive care needs volunteers from across the UK from every ethnic background who tested positive for the virus but didnÕt become really sick. And the National Institute for Health Research has established an online system where people can sign up to be contacted about participating in vaccine studies. Almost all of us have at times felt helpless in the face of this pandemic and struggled to see when it might end. This hopeful news shows we are making progress. „ Dr Charlotte Summers is a lecturer in intensive care medicine at the University of Cambridge