The coronavirus pandemic has not exactly yielded much in the way of good news. But now a trial into an experimental Covid-19 vaccine is giving us one reason to be hopeful.

Results from a team at the University of Oxford show that its vaccine—co-developed with the pharmaceutical firm AstraZeneca—is safe in humans and provokes an immune response. While this is a long way from being a fully working vaccine, it’s a promising, and vital, first step towards getting one.

Here’s everything you need to know about the Oxford study, and what it means for the future of Covid-19 vaccines.

What’s the Good News?

The Oxford team found that its vaccine provoked an immune response and didn’t produce any serious side effects. The vaccine prompted neutralising antibodies—the kind that defends cells against attack from virus—in at least nine out of ten of those who had a single dose of the vaccine. The immune response peaked 28 days after the vaccine, but remained high until day 56, which was the final day covered by this scientific paper. The study is still ongoing.

The results are from a study involving 1,077 healthy adults aged between 18 and 55. Half of the participants received the new Covid-19 vaccine, while the other half—the control group—received a vaccine against a bacterial infection. Although no serious side effects were reported, around 70 percent of participants developed either a fever or a headache, although this was lower in a sub-group of participants who took a paracetamol around the same time they had the vaccination.

At this stage, the study can’t tell us whether people who have the vaccine are protected against contracting Covid-19, but it does tell us that the vaccine is safe to use, and that it provokes an immune response.

So We Haven’t Actually Solved the Vaccine Problem Yet?

Not yet, no. But this news is still significant. A less encouraging result at this early stage would have been really bad news. Now we know that this vaccine is safe to use—and is able to kick the immune system into gear—we can start to explore whether it actually provides protection against those people who are exposed to the virus.

Luckily, that work is already underway. In Brazil, 5,000 people are being enrolled into a trial where they will receive the vaccination and track whether those who have been vaccinated become ill with the virus. A similar trial using the same vaccine is underway in South Africa. In the UK, 10,000 more volunteers are being recruited into the Oxford trial, but here the background rate of infection is relatively low so there is a chance that many people who are vaccinated won’t come into contact with the virus anyway, and its protective abilities would never get put to the test.

The study does have some serious limitations too. Over 90 percent of the participants were white, and the average age was 35. To make sure the vaccine is safe for everyone, it will need to be tested on a much wider group of people, including older people, those with other health conditions and those from more ethnically and geographically diverse backgrounds. The authors say that people from these groups are being recruited in the ongoing trials in the UK, Brazil, and South Africa.

I’m Hearing Grounds for Cautious Optimism Here

That sounds about right. We don’t yet know whether having antibodies against Covid-19 actually gives us immunity against the disease, but there are some encouraging signs that may be the case, at least in the short term. A small study on macaque monkeys found that the animals could catch Covid-19, but couldn’t be reinfected 28 days after they recovered, with another study finding that protection lasted at least five weeks.