How effective are the Covid vaccines?

11 February 2021

12:29 AM

11 February 2021

12:29 AM

Reports have filtered through this morning about Public Health England’s assessment of the efficacy of the two vaccines so far administered to the public. The results have not yet been published, but the efficacy rates quoted in the Sun suggest that the Pfizer vaccine has proved to be between 79 and 84 per cent effective at stopping symptomatic infection after two doses. After one dose – which is all that most people have had so far – efficacy is reported as 65 per cent. Among the over 80s it is very similar, at 64 per cent. No figures are given for the AstraZeneca vaccine but it is suggested that the efficacy is ‘similar’.

While the figures are encouraging, they do, nonetheless, indicate that the Pfizer vaccine could have a somewhat lower efficacy than the initial trial results suggested. An analysis of the data published by the US Food and Drug Administration in December showed that the Pfizer jab appeared to have an efficacy of 95 per cent after two doses. There was a lack of data on the efficacy of the vaccine after one jab because everyone in the trial was given a second jab after 21 days.

We necessarily shouldn’t expect the results of controlled trials to be echoed exactly by what happens in the real world. Nevertheless, the results of the PHE study, if confirmed, do suggest we might have been a little over-hopeful about the near-complete ability of the Pfizer vaccine to prevent symptomatic infection. One of the factors that will arouse acute interest is whether variant strains – in particular the Kentish variant – will have played a part in the lower efficacy rate. Pfizer has said that laboratory tests shows that the vaccine should work just as well against the Kent variant.

Moreover, the results leaked this morning do not show how well the Pfizer vaccine is at preventing asymptomatic infection. That is important, because it will govern how far and fast that Covid-19 can spread with a partially-vaccinated population.

Interestingly, the efficacy for the AstraZeneca vaccine seems to be going in the opposite direction to that of the Pfizer jab as more data comes in. Initial results showed an overall efficacy rate of 70 per cent. However, a follow-up report on the phase 3 trials published last week suggested that a single dose of the vaccine prevented symptomatic infection to the tune of 76 per cent. It also suggested that a single dose had an efficacy rate of 67 per cent in preventing any infection, symptomatic or asymptomatic.

It shows that we shouldn’t get too carried away with the initial results of trials and that the vaccines may prove to be more or less efficacious when we have much more data. But overall, the PHE data appears to show that both vaccines are well worth the efforts that have been spent developing them and getting them into people’s arms.

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