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A health worker in Germany being vaccinated with the Oxford/AstraZeneca vaccine
By early March only one-third of Germany’s stocks of the Oxford/AstraZeneca vaccine had been used – possibly because of misinformation about its effects. Sean Gallup/EPA-EFE

How well does the AstraZeneca vaccine work? An expert reviews the current evidence

When the Oxford/AstraZeneca vaccine was first authorised by the UK Medicines and Healthcare products Regulatory Agency, it was hailed as a milestone in turning the tide on the coronavirus.

But in the time since, this highly efficacious vaccine has suffered a lot of reputational damage. In January, the German press and French president Emmanuel Macron falsely claimed that it is largely ineffective in people over 65, despite there being no evidence to support this.

Yet concrete evidence on how protective the vaccine is in older people has been lacking, leading some European countries to restrict the vaccine to under-65s. Together, these factors appear to have created high levels of scepticism in Europe and low levels of uptake.

Thankfully, though, more information on how well the vaccine works is arriving all of the time – and seeing the vaccine’s positive effects, some countries are now reversing their restrictions on giving it to over-65s. Here’s what we know so far about the vaccine’s effects.

Is the Oxford/AstraZeneca vaccine as protective as the others?

Trials suggested it’s a bit less protective than other vaccines. Pfizer’s, for example, prevented symptomatic disease 95% of the time in testing, whereas the Oxford/AstraZeneca vaccine did so 70% of the time.

But recent real-world data from the UK suggests that the Oxford/AstraZeneca vaccine might actually be a bit better than Pfizer’s at preventing serious disease and hospitalisation. However, these studies haven’t been set up to avoid outside biases affecting results. For example, the Pfizer vaccine was rolled out first, so might have been given disproportionately to more vulnerable people. Also, these studies are preprints, meaning they haven’t been checked by other scientists yet.

So whether the Oxford/AstraZeneca vaccine is more or less effective than others at preventing COVID-19 isn’t clear yet. The important thing to remember is that all the authorised vaccines are safe and offer very good levels of protection against COVID-19. If you’re offered any vaccine, you should take it.

Does the vaccine work in older people?

An early stage of testing – a combined phase 1 and 2 trial – strongly suggested that the Oxford/AstraZeneca vaccine would offer protection to older people. It showed the vaccine generated just as strong an immune response in older participants as younger ones.

However, as we don’t know to what extent different parts of the immune response act against the virus, this alone didn’t prove that the vaccine would be effective in this age group. And in the final phase of testing – phase 3 trials – which are designed to prove that a vaccine is protective, there weren’t enough people over 55 to give a reliable estimate of protection for this group.

A vial of the Oxford/AstraZeneca vaccine
Several billion doses may be distributed this year. Fighting misinformation will be key to ensuring they have the greatest impact. Sean Gallup/EPA-EFE

Exactly how protective the vaccine is in older people remains uncertain. But a recent preprint suggests it works very well. A single shot of the Oxford/AstraZeneca vaccine appears to reduce the chances of people being admitted to hospital with COVID-19 by 80% after the first dose.

Does the vaccine protect against new variants of the virus?

It depends on the variant. A preprint suggests that the vaccine may be as effective against B117, the variant that has arisen in the UK, as it was against the earlier forms of the coronavirus.

But against the variant circulating in South Africa, B1351, a preprint suggests it may be much less effective at preventing mild to moderate disease than it was against previous forms of the virus. It’s believed this is because of a mutation called E484K, which changes the virus’s exterior so that existing antibodies to SARS-CoV-2 are less able to bind to it.

Sarah Gilbert, lead researcher on the Oxford Vaccine Development Programme, believes that the Oxford/AstraZeneca vaccine will still protect against severe disease when facing these variants. However, this has yet to be proven.

Variants circulating in New York (B1526) and Brazil (P1) also have this E484K mutation. We don’t yet have data on how well the vaccine works against these variants, but it’s plausible that it may struggle against them too.

In the meantime, the vaccine’s developers have confirmed that they can tweak it to handle these mutations. They estimate an updated booster could be ready by autumn.

A health worker in Egypt loading a syringe with the Oxford/AstraZeneca vaccine
The Oxford vaccine is already being widely deployed: doses made in India are now being administered in Egypt, for example. Khaled Elfiqi/EPA-EFE

Will the vaccine will give me bad side-effects?

You might experience some, but they shouldn’t be severe or last long. Common side-effects include a sore patch where the needle went into your arm, a temperature, aching, tiredness and feeling sick. These aren’t necessarily bad, because they’re a sign that your immune system has noticed the vaccine and is responding to it.

In France and Germany, reports of healthcare workers experiencing flu-like symptoms after receiving the vaccine – at rates of 40% of people or higher – have worried people. However, these effects were also seen in the vaccine’s combined phase 1 and 2 clinical trial. Researchers found that taking paracetamol helped, and that these effects had largely subsided after seven days.

Is delaying the second dose risky?

The intervals between doses have been a bit controversial, since the 12-week gap that’s being left between them in the UK is different from the dosing schedule that was submitted for regulatory approval by the manufacturer.

However, it looks like delaying the second dose to 12 weeks after the first is actually a good idea. The protection from the first injection seems to last, and if you have the second vaccine 12 weeks later, rather than four weeks as originally proposed, that seems to generate a stronger immune response.

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