New research suggests that the covid-19 vaccine developed by AstraZeneca and Oxford University not only protects against the symptoms of the viral disease, but also lowers a person’s risk of transmitting the virus to others. While scientists have been optimistic that these and other covid-19 vaccines would lower transmission risk, the new data is some of the first direct evidence to support that assumption.
Researchers at Oxford University in the UK released the data this week in one pre-printed paper for the Lancet. Preprints are preliminary research papers that have not yet gone through the formal peer review process, so any conclusions should be viewed with some extra caution. Regardless, the findings of their study – an analysis of longer-term data from clinical trials of the vaccine in the UK and Brazil – seem encouraging.
First, the results suggest that even a single dose of the Oxford / AstraZeneca vaccine may be 76% effective in preventing coronavirus disease 21 days after receipt (it is thought that it takes 21 days for the immune system to have an effective response built against the virus). Importantly, the effectiveness did not seem to diminish until three months later. The effectiveness of a two-dose regimen after three months was slightly higher, about 82%. It is possible, the researchers think, that doses three months apart may be the optimal schedule, since effectiveness was lower in those who received the doses only six weeks apart. As other research has shown, no coronavirus-related hospitalizations were found in those vaccinated after 21 days, and the vaccine was well tolerated.
The longevity of the protection provided by a single dose is especially reassuring, as the UK has allowed doctors to delay people with the second dose for up to 12 weeks or about three months to extend the limited vaccine supply. Some scientists were concerned that a delayed delivery strategy would result in weakened or short-lived protection for some, and countries, including the US, have so far opted not to follow the UK’s lead for their approved vaccines, citing to a lack of evidence for its safety. While other data needs to be collected to confirm this trend, including from other two-dose courses, this is a sign that a delayed dose may not be too risky after all, at least for the Oxford / AstraZeneca vaccine.
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Many scientists have carefully noted that these vaccine studies have so far only clearly shown that they are effective in preventing disease from the virus, not in preventing transmission from a vaccinated person to an unvaccinated person. Still other experts have argued that it is very unlikely that a vaccine would lower the risk of the former, but not the latter. Although there have been some early proof that other vaccines, such as the Pfizer / BioNTech injection, do indeed reduce transmission rates, the Oxford / AstraZeneca vaccine trials have explicitly tested for that possibility since the beginning.
Volunteers in the UK trial were regularly swabbed for the virus through PCR tests. Compared to the volunteers who received a placebo injection who later became infected and identified through testing, the vaccinees were, on average 67% less likely to have noticeable levels of the virus in their system, meaning infection. A lower risk of infection in vaccinated people should too means they are less likely to transmit the virus. Again, this is something that more data from other studies and vaccines will have to confirm. But if true, it would of course be great news to end the pandemic as soon as possible.
There are still questions about how the spread of new virus variants will affect the effectiveness of these vaccines (so far it is believed that variants are unlikely to completely escape vaccine immunity). Science aside, the supply and distribution of these vaccines remains suboptimal, especially to lower income countries. But there are also signs of improvement.
Last week, the EU authorized the emergency use of the Oxford / AstraZeneca vaccine, following the UK approval last December. The U.S. Food and Drug Administration is also expected to decide whether the single-dose Johnson & Johnson vaccine will be approved in the coming weeks (a decision on the Oxford / AstraZeneca vaccine is likely to follow after the end of the U.S. trial in the U.S. spring). And just today, COVAX, an organization supported by several deliberate public health groups and the United Nations, announced his distribution forecast for the AstraZeneca / Oxford vaccine.
In the first and second half of 2021, it plans to distribute more than 330 million doses to 145 countries, along with 1.2 million doses of the Pfizer / BioNTech vaccine. These shipments are expected to cover only about 3% of the people in these countries, but it is hoped that this will be enough to cover some of the most vulnerable groups, such as health workers.