A study from the University of East Anglia (UEA) reported that the Pfizer vaccine provides “very high” protection after a single dose.
The researchers drew on Israeli data, where the vaccine had been widely administered, and found that the vaccine was 90% effective at 21 days after the initial dose. This supports the plan that the UK and other nations have of delaying a second dose to achieve maximum coverage. However, they also noted that infection rates increased eight days after the first dose, which they attribute to people becoming less cautious as a result of the vaccination. The study is available on the medRxiv preprint server, and has not been peer reviewed, as it is a rapid response to the ongoing COVID pandemic.
Lead researcher and COVID expert Prof Paul Hunter, from UEA’s Norwich Medical School, said: “A second dose of the Pfizer vaccine would normally be given 21 days or more after the first to top up and lengthen the effect of the first dose.
“But here in the UK, the decision was made to delay the timing of the second injection until 12 weeks after the first.
“The logic behind this is to protect more people sooner and so reduce the total number of severe infections, hospitalisations, and deaths.
“But this decision caused criticism from some quarters due in part to a belief that a single injection may not give adequate immunity.”
Prof Hunter explains the motivation for the study was previous flawed research on the Pfizer vaccine, also using Israeli data. But the study did not consider effectiveness past day 18.
The researchers observed that case incidence rose up til day eight by which time it had doubled, then fell. Prof Hunter said: “We found that the vaccine effectiveness was still pretty much zero until about 14 days after people were vaccinated. But then after day 14 immunity rose gradually day by day to about 90 percent at day 21 and then didn’t improve any further. All the observed improvement was before any second injection.
“This shows that a single dose of vaccine is highly protective, although it can take up to 21 days to achieve this.”
Although the vaccine’s effectiveness beyond this is not known, it still supports the UK’s decision to space out vaccine doses, Prof Hunter concluded.
Source: Medical Xpress
Journal information: “Estimating the effectiveness of the Pfizer COVID-19 BNT162b2 vaccine after a single dose. A reanalysis of a study of ‘real-world’ vaccination outcomes from Israel” is published on the medRxiv pre-print server: www.medrxiv.org/content/10.110 … 021.02.01.21250957v1
