New Israeli data show good protection after one dose of Pfizer vaccine

New data published yesterday in the Lancet appeared to show that there is very good protection from becoming infected with Covid-19 two weeks after one dose of the Pfizer vaccine.  In this post I describe the results, what are their limitations and what they mean.  I also relate the results to similar data from AstraZeneca. 

What did we know about protection after one dose of the Pfizer vaccine before this study?

  • The planned Pfizer vaccine dosage required two doses 21 days apart (although in practice this may extend to 28 days) and the main published clinical trial only reported on the number of cases in the trial occurring from two weeks after the second dose
  • The UK government, as a matter of policy, delayed the second dose for those given the Pfizer vaccine to 3 months – which raised the question as to the protection people would have during that interval between doses
  • Pfizer provided the UK with previously unpublished data from their trial
    • Pfizer looked at how many cases had occurred from 15 days after the first dose but before the second dose 
    • They showed that the numbers of cases in that narrow window of the interval 15-28 days between doses was very different in the vaccine and placebo groups.
  • I had not seen these data before today but they will have provided reassurance to the UK that delaying the second dose will still provide some protection
  • I will repeat this again in this post but 
    • We do not know if this apparent protection after one dose will be sustained beyond 28 days
    • The agreed vaccine schedule for this vaccine requires 2 doses

The new Israeli data

  • It is widely recognised that Israel has led the world in the proportion of the population it has vaccinated 
  • The Pfizer vaccine has been used throughout 
  • A priority target was health care workers, in whom the vaccine programme started on December 19th 2020
  • The design of this study was epidemiologically a clever one and shown in the diagram below
  • They studied all the 9109 eligible health workers in this target group
    • By 24th January 2021 some had had 2 doses, some 1 dose, and others were still waiting for their first dose
    • All workers though provided daily reports on their health to find out if they had developed Covid-19
    • It was assumed that it might take 14 days for the first vaccine dose to have any effect so the research was interested in how many cases occurred in the 3 time intervals from the 19th December 
      • Before they had their first jab (the pink bar)
      • From after their first jab until 14 days later (the yellow bar)
      • From 15 days after until their second jab (the green bar)
    • Most of the infections they captured were from the workers’ reports of symptoms, some were picked up by active contact tracing and were asymptomatic
    • This is what they found:
  • The rate of all cases  – blue bars – (both symptomatic and asymptomatic) fell from the pre-vaccination period by 30% in the 1-14 day period after the first dose and by 75% in the 15-28 day period after the first dose but before the second dose
  • The rate of just cases with symptoms – orange bars – fell from the pre-vaccination period by 47% in the 1-14 day period after the first dose and by 85% in the 15-28 day period after the first dose but before the second dose


  • This is good news, but as with the original trial we do not know if these reductions will be sustained 
  • Even during this period of observation, there may have been differences in how the participants behaved in terms of social distancing (but they were health workers) so perhaps had no choice)
  • Different strains may have been predominant at different periods (but note  also from the design of the study that, for example, some workers only in the pre-vaccine stage for the first week(s) of the study period, whereas others were still waiting for their vaccine and would have contributed more days of pre-vaccination)


  • These data do not prove that one dose of the Pfizer vaccine will be enough, and in Israel as in other countries the two-dose regime will continue
  • The data are reassuring that whilst waiting for a second dose, although social distancing etc should be maintained, for occupationally exposed groups like health and care workers there is reasonable protection

And for AstraZeneca vaccine recipients

  • We have more data on the protection after the first dose from AstraZeneca because of their clinical trial 
  • I discussed these data only recently (
  • Interestingly that report showed a reduction in symptomatic cases after the first dose of around 75% and of all cases by 67%: results that I feel are comparable to these new Pfizer data


  • The data now from these two vaccines show a good reduction in both symptomatic and all cases after the first dose.  
    • For AstraZeneca we know that this persists for 3 months
    • For Pfizer, the length of the protection is not yet known but hopefully results from the UK data will give a similar answer when they are become available over the coming weeks
    • This is of relevance for countries like the UK* with a longer interval between doses

*EMA (European Union) allowing up to 6 weeks and probably will be the interval to be adopted in Denmark and the Netherlands

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