One Pfizer/BioNTech Vaccine Dose May Confer 90% Immunity Against Covid After 21 Days

We recently received encouraging indications that the Oxford/AstraZeneca COVID-19 vaccine not only protects against disease, but may also reduce transmission of the virus. Now comes a study suggesting that Pfizer/BioNTech’s mRNA vaccine may be far more effective after a single dose than was originally thought.

Pfizer themselves originally quoted an efficacy of 52% following a single dose, with some data from Israel last month suggesting that the number could be even lower. However, a reanalysis of this data paints a rather different picture.

According to the newer research (which has not yet been peer reviewed), the original study’s approach included a number of flaws, such as not estimating vaccine effectiveness beyond 18 days post-vaccination. The present study reported that infection rates actually increased following vaccination (possibly due to people behaving less cautiously), roughly doubling by day 8 before starting to decline. They report that efficiency was around 0% at day 14, but rose to around 90% at day 21 before levelling off.

If true, this finding is reassuring for the UK and other countries that have chosen to delay the second dose in favour of prioritising the first. The UK’s decision to delay the second dose by 12 weeks initially came under criticism due to the fear that a single dose would not offer adequate protection. This new research suggests that after the first 3 weeks, a single dose may be sufficient to reach high levels of protection. While we still don’t know how long this protection lasts, it is unlikely to decline significantly in the following 9 weeks before the second dose is received.

This study also highlights the importance of minimising potential exposure to the virus following vaccination, as it appears to take around 2 weeks before immunity starts to develop, while maximum efficiency takes up to 3 weeks.