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expert reaction to media reports of study results from Israel and comments made by Prof Nachman Ash, Israel’s coronavirus tsar, about the efficacy of a single dose of the Pfizer vaccine

There have been media reports of results from a serological study conducted at Israel’s Sheba Medical Centre suggesting Pfizer/BioNTech vaccine may prevent COVID-19 transmission, and comments made by Prof Nachman Ash suggesting a single dose of Pfizer/BioNTech vaccine appeared “less effective than we had thought”.


Prof Stephen Evans, Professor of Pharmacoepidemiology, London School of Hygiene & Tropical Medicine, said:

“It is vital that advice and policy take into account the latest available data. However the reports that have come from Israel are insufficient to provide any evidence that the current UK policy in regard to delaying the second dose of vaccines is in any way incorrect.

“The reported efficacy of one dose has not been compared using the same methods and patients with the efficacy of two doses at 84 days. The details of the different studies have only been released, it seems, at a press conference the reasons for which are unclear. There is a need for at least a preprint giving the detailed methods and data to understand and interpret these findings. This applies especially to the reported value of 33% efficacy from the Clalit-based study. This study seems to have been registered at on 14th January, though it should be emphasised it is not a randomised trial.

“It is not sensible to compare efficacy derived from an observational study of this type which is subject to many biases, with the efficacy derived from randomised trials.

“It is also important to recognise that efficacy against detected COVID-19 may be less important in the UK context then efficacy against hospitalisation and death. It was suggested from the randomised trials that efficacy against hospitalisation was greater than that against detected COVID-19 cases.

“The UK will soon have its own data showing efficacy after the first dose for the different vaccines currently in use and any policy changes should await more robust data. If for example the efficacy after one dose was 33% but the efficacy after two doses was 60% the UK policy would still be justified. Similar arguments apply in relation to efficacy against serious disease.”






All our previous output on this subject can be seen at this weblink:



Declared interests

Prof Stephen Evans: “No conflicts of interest.  I am funded (one day per week) by LSHTM.  They get funding from various companies, including Astra Zeneca and GSK but I am not funded by them, I have no involvement in obtaining funding from them and I am not an investigator on any grants obtained from them.  I am the statistician to the ‘meta-Data Safety and Monitoring Board’ for CEPI.  I am paid for my attendance at those meetings and will be paid expenses for travel if that occurs.  I am a participant in the Oxford/Astra Zeneca trial, and on 13th January 2021 learnt I had received the active vaccine”

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