On Monday, researchers in the UK became the latest to announce promising results from their experimental vaccine candidate against covid-19. In the data seen so far, the vaccine appeared to be at least 70% effective at preventing infection from the coronavirus, though it may be up to 90% effective with a certain dosage. Its developers, which include pharmaceutical company AstraZeneca, will now submit the vaccine for review by regulatory authorities in the UK and other countries.
The vaccine was developed through a collaboration between the University of Oxford and AstraZeneca. It works by packing genetic instructions on proteins from the coronavirus inside another, neutered virus—in this case, an adenovirus. Once introduced into the body, the dummy virus infects cells and compels them to produce these proteins (the virus is altered to be incapable of replicating itself like normal, rendering it harmless). These proteins will then hopefully provoke a strong-enough response that trains the immune system on how to fend off a real infection from the coronavirus. The vaccine is provided through two doses, taken one month apart.
The data presented on Monday is said to be from over 11,000 people given the vaccine, who were compared to a similar-size group given a control vaccine and placebo (the control vaccine protects against bacterial meningitis). The results are from two separate trials in the UK and Brazil.
Out of these participants, 131 had developed covid-19 when the analysis was completed. Across the UK and Brazil trials, 101 people in the control group caught the infection, compared to 30 in the vaccine group. That translates to a vaccine with roughly 70% effectiveness. There were also no reports of serious illness from covid-19 in the vaccinated group, as well as no serious safety concerns confirmed to be related to the vaccine. Earlier this year, a panel of outside experts determined that there wasn’t enough evidence to conclusively link two reports of neurological illness among trial participants to the vaccine.
“Today marks an important milestone in our fight against the pandemic. This vaccine’s efficacy and safety confirm that it will be highly effective against COVID-19 and will have an immediate impact on this public health emergency,” Pascal Soriot, the CEO of AstraZeneca, said in a statement.
As with the results previewed by companies Pfizer and Moderna on their respective vaccine candidates, the data here has to be yet studied in full by the scientific community, so the news should be taken with some caution. And there are a few added quirks that will make understanding the potential for this vaccine more difficult for the time being.
For one, people in the UK received a different dosing regimen than people in Brazil. They received half the dosage of the vaccine in the first shot, then a full strength shot the second dose. In these volunteers, the vaccine’s effectiveness was 90%, compared to 62% effectiveness in the Brazil trial.
According to the researchers, this disparity might be explained by a known limitation of these types of vaccines, which are called vector vaccines. Put simply, giving someone too high a first dose of the dummy virus could prime the immune system to defend itself too well against it, which then limits the effectiveness of a second dose given later (a similar problem is that a person’s preexisting immunity to other adenoviruses could limit an adenovirus-based vaccine’s potency). So it’s possible that the dosage used in the UK trial may be the more effective method to use going forward.
That said, the UK data right now comes from a much smaller sample of volunteers than the data from the Brazil trial, though this gap might not be as big or exist at all by the time of a final analysis. There may also be other factors that explain the effectiveness variation, such as population-level differences between people in Brazil and the UK.
On the flip side, these results could be underselling themselves, compared to what we’ve seen from other vaccine trials. The AstraZeneca/Oxford trials were designed to look for both asymptomatic infections and symptomatic infections. This was done by asking volunteers to take weekly swab tests and also included people who felt sick enough to see a doctor and get confirmation that way. The Pfizer and Moderna trials, however, focused only on symptomatic cases where people were sick enough to want testing. It’s possible that AstraZeneca’s vaccine is overall more effective at preventing either type of infection—we just don’t have the data to know right now.
Another unanswered question for all of these vaccines is how effective they are at preventing transmission of the virus. That’s important to find out, because someone who is vaccinated could possibly still carry enough of the virus once infected to spread it to other unvaccinated people for a brief time, even if the vaccine succeeds in keeping that person from becoming seriously ill. Given that it will take months if not years to vaccinate much of the world’s population, it’s a crucial potential risk we have to understand better. And of course, we don’t know how long any vaccine-provided immunity will last, though there are encouraging signs that natural immunity may stick around for possibly years for most people.
Caution aside, this is yet another piece of good news for the long-term outlook of the pandemic. AstraZeneca/Oxford’s vaccine is the first of its kind to reach this stage of development, yet there are other candidates close behind. Unlike Pfizer’s mRNA vaccine, vector vaccines only need standard refrigeration, not extreme cold temperatures, to be kept viable. Moderna’s mRNA vaccine is said to have overcome this limitation, but AstraZeneca/Oxford’s vaccine is based on long-existing technology that’s expected to make mass production more affordable. The University of Oxford has said that it has plans to mass-produce the vaccine in 10 countries the moment it’s approved for use, and it has reached an agreement with AstraZeneca to provide the vaccine on a not-for-profit basis to low- and middle-income countries for as long as needed.
Early reports suggest the UK may dole out an emergency approval for the vaccine as early as next week, with India following close behind. In the U.S., meanwhile, the Food and Drug Administration has now scheduled December 10 to have an advisory committee look over the data from Pfizer and possibly Moderna’s candidate and recommend whether they should get emergency approval. If all of this goes well, the first batch of covid-19 vaccines could start to reach high-risk members of the public, such as health care workers, by year’s end.