For the first time, a peer-reviewed trial of a vaccine against Covid-19 has shown recipients produce antibodies against the virus. Side-effects were common in the phase 1 trial, but not severe. Larger trials have started, or soon will, to address the many questions this study could not, but for the moment the news is about as good as it could be, given the limitations of the testing so far.

More than 100 teams around the world are rushing to develop vaccines against Covid-19, many of which have already entered clinical trials. However, some have rushed to report positive findings that have not been peer-reviewed, and often lack details, making it hard to assess their validity. In other cases, encouraging signs have been recorded from studies on animals, which while a good start, won't necessarily translate to humans.

Against that background, a paper in the New England Journal of Medicine stands out. It reports on a trial of 45 healthy adults aged 18-55, each vaccinated twice, 28 days apart, with the mRNA-1273 vaccine developed by the National Institute of Allergy and Infectious Diseases (NIAID) and pharmaceutical company Moderna. Instead of a control group, participants were divided into three streams, given 25μg, 100μg, or 250μg doses respectively.

All participants subsequently produced antibodies against the SARS CoV-2 virus, and their blood serum showed neutralizing activity against the virus, preventing it from entering cells. Moreover, the paper reports, “After the second vaccination, serum-neutralizing activity ...[showed] values generally similar to those in the upper half of the distribution of a panel of control convalescent serum specimens.” In other words, the vaccine appears to provide stronger protection against new infections than an average bout of actually catching the disease.

Side effects included “Fatigue, chills, headache, myalgia, and pain at the injection site,” but none proved long-lasting. Side effects were greater with the larger doses, but so was the antibody response, so there will be a trade-off in finding the best quantity to use. Lead author Dr Lisa Jackson of Kaiser Permanente Washington Health Research Institute told Time: “It seems that two doses are needed, which is what is expected from this type of vaccine and from a vaccine against an emerging virus that hasn’t been present in the population. The immune system needs to be set up by the first dose to then respond more vigorously to the second one.”

Three major unknowns are how much protection these antibodies actually offer against infection, whether this extends to more vulnerable older populations, and how long any benefits last. To answer those questions, 30,000 people will start being vaccinated from July 27, with half getting a placebo. Unusually, this phase 3 trial will overlap with a phase 2 trial of 300 participants already underway, including some at higher risk from the virus, who will be studied in greater depth.