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expert reaction to study looking at COVID infection risk as time increases after second Pfizer vaccine dose in Israel

A study published in The BMJ looks at time since receiving the Pfizer-BioNTech COVID-19 vaccine and risk of SARS-CoV-2 infection.


Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice, Immediate past Chair of the BMA Public Health Medicine Committee, said:

“The press release is a fair representation of the paper’s findings, and the scientific quality of the study is high.

“The paper is useful, partly inasmuch as we have been aware that immunity, following vaccination with two doses of Covid vaccines, wanes over time; but much of this knowledge has come from reviewing surveillance data.  Relatively few academic papers have reported on this.

“The fact that immunity wanes over a matter of just a few months matters, particularly with regard to transmission rates.

“Note that the paper’s main outcome measure is testing positive for the virus.  We already know that the vaccines are most effective at preventing serious illness and death, less effective at preventing milder symptomatic disease, and less effective again at preventing a- or paucisymptomatic infection (few or no symptoms).  We also know that immunity to serious illness depends to a large extent on cellular immunity, which is likely to be longer-lasting than humoral immunity (antibody levels); whereas as resistance to infection per se depends more on humoral immunity.  (I explain this in more detail here1.)  This means that this paper does not provide information about immunity to different levels of infection.

“This means that it doesn’t necessarily predict an increase in death, hospitalisation, or critical care admission rates – just an increase in infections.  But this increase in infections indicates an increased likelihood of transmission, including to people who are not [adequately or at all] protected through vaccination.

“Note that the population in Israel received almost exclusively the Pfizer BioNTech BNT162b2 mRNA vaccine.  The findings of this study may apply equally to other vaccines – perhaps particularly, to similar (eg mRNA) vaccines; but we cannot assume that without further study.

“The study was also undertaken in a population in which the second dose was routinely given four weeks after the first dose.  Note that we now have good evidence that the longer interval between first and second doses used for most people in the UK generates better, probably longer-lasting immunity.  It is not unlikely that immunity in the UK would not fall as far, as soon, for this reason.

“The study was also limited to people aged 18 years or more.  We cannot be sure if immunity wanes at a similar rate in younger people.

“The findings that immunity wanes over as little as three months is further support for the idea that a third dose of vaccine will routinely be needed both to provide better protection both against serious disease and, importantly, if we are to get close to or achieve herd immunity, against infection and transmission.”




‘Elapsed time since BNT162b2 vaccine and risk of SARS-CoV-2 infection: test negative design study’ by Ariel Israel et al. was published in The BMJ at 23:30 UK time on Wednesday 24 November 2021.

DOI: 10.1136/bmj-2021-067873



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



Declared interests

Dr Peter English: “Dr English is on the editorial board of Vaccines Today: an unpaid, voluntary, position. While he is also a member of the BMA’s Public Health Medicine Committee (and its chair until Oct 2020), this comment is made in a personal capacity. Dr English sometimes receives honoraria for acting as a consultant to various vaccine manufacturers, most recently to Seqirus.”

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