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What the Lancet study tells us about the Oxford vaccine

9 December 2020

8:46 AM

9 December 2020

8:46 AM

While the Pfizer vaccine became the first to be used in a public vaccination programme on Tuesday, the Oxford/AstraZeneca vaccine team became the first to publish their results in a peer-reviewed journal, the Lancet.

As the press release announcing the results explained, the overall efficacy rate of the Oxford vaccine was measured at 70 per cent, but that concealed a large difference between different arms of the trial.

When people were given two standard doses of the vaccine, its efficacy rate was only 62.1 per cent. Yet intriguingly, in one group which was given a half dose followed by a standard dose, the vaccine had an efficacy rate of 90 per cent. The Oxford vaccine team could not explain this difference and the Lancet paper says there is ‘a possibility that chance might play a part’.


There is also the question of age. All the participants who received the half dose followed by a full dose were under 55. Given that the death rate for Covid-19 rises steeply with age, it is very unsatisfactory that any data should be limited to the under-55s – a point which the group has admitted. Further trials are now needed.

In the meantime, the Medicines and Healthcare products Regulatory Agency (MHRA) might be minded to approve the vaccine for use only in the under 55s, or the government might decide only to use it in younger age groups. Given that the AstraZeneca vaccine will likely be cheaper than the Pfizer and Moderna alternatives, and it is much easier to store and transport, there will be a temptation to concentrate use of the first two on the elderly and other vulnerable groups, and use the AstraZeneca vaccine as a rapid way to mop up the rest of the population.

The overall efficacy rate of 70 percent is within the range that would have been considered acceptable before the Pfizer and Moderna vaccines (both of which claim efficacy of around 95 per cent) but it is somewhat disappointing. The Lancetauthors cite a US study which sought to assess what level of efficacy would be required for a vaccine to stop the Covid epidemic in its tracks, which found that:

‘a vaccine with efficacy of 60-80 per cent could allow reduction in physical distancing measures, but this will still require high coverage.’

The public though is unlikely to be satisfied with a mere reduction in social distancing – there is a high expectation that a vaccine will mean an end to social distancing for good, allowing a line to be drawn under the epidemic.

The Lancetstudy also shows that the AstraZeneca vaccine is a lot more effective when there is at least a six-week gap between the two jabs – when two standard doses were delivered less than six weeks apart the efficacy rate fell to 53.4 percent, against 65.4 percent when the gap was six weeks or more. That suggests, too, that it could take longer to roll out the vaccine compared with the Pfizer vaccine, where two doses are being given three weeks apart.

The Lancetpaper notes that there were four deaths among participants in the trial, though none was linked to the vaccine – indeed, three of them occurred in the control group. One death was from fungal pneumonia (it doesn’t state whether that was among the control group or the vaccine group), one was a road accident, was a homicide – showing perhaps an occupational hazard of conducting a trial in Brazil – and the other from ‘blunt force trauma’.

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