EXPLAINED: AstraZeneca's COVID-19 vaccine efficacy and expiry dates

How can government extend a vaccine expiry date? Why didn't government know sooner it wasn't working? Your AstraZeneca vaccine questions answered.

A handout picture released by the University of Oxford on 23 November 2020 shows a technician working on the University's COVID-19 candidate vaccine, co-invented by the University of Oxford and Vaccitech in partnership with pharmaceutical giant AstraZeneca. Picture: John Cairns/University of Oxford/AFP

JOHANNESBURG - In a statement released by Wits University on Sunday afternoon, trial investigators said that the AstraZeneca vaccine had high efficacy against the original coronavirus for people who had mild to moderate COVID-19 symptoms. But 501Y.V2 - the second variant of the virus first sequenced in South Africa and now the dominant variant in the country - did not see such positive results.

READ: SA halts rollout of AstraZeneca vaccine over efficacy against second variant

According to the university, more than 2,000 South African adults between the ages of 18 and 65-old-years old were enrolled from June to November across seven sites in Gauteng and the Western Cape.

In a pre-print of the ChAdOx1 nCoV-19 (the name of the AstraZeneca vaccine) study, researchers said in young South African adults, the vaccine offered limited protection against the 501Y.V2 variant.

This means that even if you're vaccinated after testing positive for the first variant of the coronavirus, it is still a possibility you could get infected with the second variant as the virus mutates. The vaccine was developed for the first variant and not for the mutation. This is also why population or herd immunity should be achieved as soon as possible.

But how does this work? Should we be worried that we have an ineffective vaccine? And why didn't government know some of these issues sooner? Here are some important things to note about the AstraZeneca vaccine.


It is not fake, it works - it just doesn’t work as well as we hoped it might against the new variant. That’s quite an important point. The study conducted was small, and the investigators in the study believe it might offer more protection against more severe forms of the disease.

AstraZeneca’s vaccine has lower efficacy than Pfizer or Moderna’s, but is still important. It is the cheaper of the options available - it costs $4 per dose, compared to $20 for the Pfizer vaccine and between $10 and $50 for Moderna's. It is also easier to transport to South Africa, as it doesn't need cold storage to remain alive. It is not a fake vaccine. Having some form of efficacy against one variant is better than having none at all.

We're not the only ones using it. Through COVAX, most United Nations countries, as well as Palestine and the Vatican, came together to procure vaccines for the virus. This includes the US, which joined late under new President Joe Biden. AstraZeneca is one of the vaccines these countries are buying. Outside of COVAX, AstraZeneca is already the used vaccine in the United Kingdom and will continue to send two million doses a week there, and plans to send 400 million doses to the European Union states.


According to scientists, all vaccines have shown difficulty in fighting off the second variant of COVID-19. Sinopharm and Pfizer's have seen a minimal decline in efficacy, while AstraZeneca's has shown a substantial decline.

The AstraZeneca vaccine trials set out to gauge its efficacy to prevent mild to moderate COVID-19 symptoms.

As Wits explained in its release on Sunday, 2,000 South African volunteers were used in its trial. Twenty-two percent of that group had success with the vaccine. We are still waiting to hear about further trials as that data is not available yet. People were between the ages of 18 and 65 and this did not include people with HIV. Data on whether the vaccine provides protection against severe forms of the disease, hospitalisations or deaths are not available.

According to health journalist Laura Lopez Gonzalez, "the study only had enough people to be able to show if the vaccine was at least 60% effective and could not say if it protected against severe COVID-19. A bigger, phase 3 study would be needed for that."

Clinical trials of the single-shot Johnson & Johnson vaccine have proven it to be effective in preventing hospitalisation and death. The J&J vaccine was initially trialed with 45,000 people, 15,000 of which were South African, and included people with comorbidities such as diabetes and HIV.

"The South Africa AstraZeneca trial took place during the tail end of the first wave & during the second wave, which was fueled by the new variant first discovered in South Africa. The vaccine was designed to address the original virus, and before the new variant was discovered," Gonzalez explained on Twitter. "Until the end of October 2020, the AstraZeneca vaccine was able to reduce a person's risk of COVID-19 becoming infected with the virus just 14 days after injection but this changes as the new variant dominates infections in South Africa

Novavax was effective in 49% of people trialed with mild to moderate symptoms. Meanwhile, Sinopharm's vaccine proved to be effective in lab trials, according to the health presentation on Sunday evening.


The AstraZeneca vaccine has a six-month expiry period. But it is still early days of our tests. According to public health specialist Kerrin Begg, the reason is because the vaccines we currently have only have study data for six months. As stability data becomes available for longer periods, so the expiry dates can be extended by manufacturers. We will have a better understanding of timelines around its efficacy.


The speed at which we've had to manufacture, test and distribute a vaccine worldwide is unprecedented. The only information we have is from day-to-day trials - everything we are learning is happening as we are dealing with the virus. It is impossible to anticipate any of this information as tests are being conducted on the go while the virus is mutating. This form of trial and study is how most vaccines are created. Scientists confirmed on Sunday that even though the vaccine wasn't as effective in mild to moderate symptoms, the vaccine could still be effective in preventing more severe COVID-19 cases.

Andrew Pollard, professor of Pediatric Infection and Immunity and chief investigator on the Oxford vaccine trial, said through the Wits statement: “This study confirms that the pandemic coronavirus will find ways to continue to spread in vaccinated populations, as expected, but, taken with the promising results from other studies in South Africa, such as those using a similar viral vector, vaccines may continue to ease the toll on health care systems by preventing severe disease.”

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