A UK doctor , Dr John Campbell, has stated that the time between the first and second doses of the Pfizer vaccine is the main driver in how effective the vaccine is for symptomatic infection.
Dr John Campbell comes to this conclusion when comparing the efficacy numbers between Israel and the UK.
While the effectiveness of the COVID19 vaccine in Israel drops off markedly each month after the second dose he claims that the UK, where the time between doses was much larger, (3 weeks in Israel and 11 weeks in the UK) the efficacy of the vaccine does not drop off like in Israel.
Does his assertion stand up to scrutiny?
Looking at ourworldindata we can see that the number of people who have received two doses of the vaccine, plateaued around the 26th of March at 55% in Israel.
Dr John Campbell quotes a 14% efficacy rate against symptomatic infection for those vaccinated in January, 44% for February, 67% March and 75% April.
After April the vaccination numbers in Israel is pretty much flat.
In summary, as Israel were ending their vaccination program (March 26) the UK was starting (March 28th). Israel are around 3 months ahead of the UK vaccination program.
Is it fair to say the time between vaccines is the prime factor in vaccine efficacy?
Unfortunately Dr Campbell does not give the efficacy rate over time for the UK, in short, it is impossible to hypothesise if his theory has any merit at this time.
Assuming we did have efficacy figures from the UK and assuming the numbers were high, I think it would be premature to make any comparisons between Israel and the UK on the efficacy of the vaccines at this time.
It is now only around 4 months since meaningful numbers of people have been fully vaccinated in the UK. In Israel it has been 6 months.
The noticeable drop off in efficacy in Israel came after 3 months.
For the time between doses theory to be analysed, we have to postpone our analysis at least for another month ie until we have data from the UK. Only when we can see the efficacy numbers in the UK for people who have been vaccinated for over 3 months can be start to compare numbers with Israel.
As things stand at the moment I think the only sensible hypothesis we can come to about the Pfizer vaccine efficacy (in regards to symptomatic infection) is that, at the moment, it appears that as time goes on efficacy drops.
And to confirm if this is true or not, by comparing UK and Israel data, we need to wait at least one more month and then analyse that month’s cases in the UK to determine how many of the cases are from people who are fully vaccinated.