UK Study Shows Similar, Significant Reductions In COVID-19 Infections With Single Dose Of Oxford-AstraZeneca Or Pfizer-BioNTech Vaccine
One dose of either vaccine resulted in 65% reduction of infections. There was greater impact against symptomatic infections (72%) than infections without reported symptoms (57%). 2 doses were even more effective against symptomatic infections (90%, and gave similar level of protection as prior infection.

COVID-19 infections fell significantly – by 65% percent – after a first dose of either the Oxford-AstraZeneca or the Pfizer-BioNTech vaccine in a study of more than 373,000 British residents who received one of the two jabs. 

The first in a series of two new studies, published by the University of Oxford, found that the reduction in new COVID-19 infections was similarly dramatic following either the first dose of the Oxford/AstraZeneca or Pfizer-BioNTech vaccine.

“We found very similar, significant reductions in infections [rates] after the first rounds of either [Oxford/AstraZeneca] and [PfizerBioNTech] vaccines,” said Dr Koen Pouwels, senior researcher in Oxford University’s Nuffield Department of Population Health, during the press briefing.

The studies, which used data from the COVID-19 Infection Survey, a partnership between the University of Oxford, the UK’s Office of National Statistics (ONS), and the Department for Health and Social Care (DHSC), analyzed 1,610,562 test results from nose and throat swabs taken from 373,402 study participants between 1 December 2020 and 3 April 2021. 

First Study Focused on Infection Reduction Rates
Clockwise, left to right: Dr David Eyre, Dr Sarah Walker, moderator Fiona Lethbridge, Dr Koen Pouwels

The first study focused on infection reduction rates following a single shot of either Oxford/AstraZeneca or Pfizer-BioNTech vaccines. Twenty-one days after a single dose of either vaccine, with no second dose, the rates of all new COVID-19 infections had dropped by 65%, symptomatic infections by 74%, and infections without reported symptoms by 57%. 

One dose of either of the two vaccines also were similarly effective against the B.1.1.7 variant, which was first identified in the UK. Vaccination was just as effective in individuals aged 75 or older with underlying health conditions as it was in those under 75 or without health conditions. 

Two doses of the Pfizer-BioNTech vaccine, meanwhile, reduced asymptomatic infections by 70% and symptomatic infections by 90%.

During the press briefing, Dr Sarah Walker, Professor of Medical Statistics and Epidemiology at the University of Oxford and Chief Investigator and academic lead for the COVID-19 Infection Survey, stated that she was ‘pleasantly surprised’ by these results. 

“The benefits are greater for people with high viral load and for people with symptoms, both of who have probably got the greatest chance of transmission, was really not necessarily something I was expecting.” 

However, too few people had yet received two doses of Oxford-AstraZeneca to assess the final degree of protection obtained from that vaccine, the researchers concluded.

In the press release, Dr Koen Pouwels said: ‘The protection from new infections gained from a single dose supports the decision to extend the time between first and second doses to 12 weeks to maximise initial vaccination coverage and reduce hospitalisations and deaths.”

“However, the fact that we saw smaller reductions in asymptomatic infections than infections with symptoms highlights the potential for vaccinated individuals to get COVID-19 again, and for limited ongoing transmission from vaccinated individuals, even if this is at a lower rate. This emphasises the need for everyone to continue to follow guidelines to reduce transmission risk, for example through social distancing and masks.”

Antibody Responses High After Second Pfizer Vaccination 
Antibody responses after receiving either AstraZeneca or Pfizer COVID-19 vaccines

The second study compared how antibody levels changed after a single dose of either Oxford/AstraZeneca or Pfizer-BioNTech vaccines, as compared with two doses of the Pfizer-BioNTech vaccine, generally given 21-42 days apart. 

Antibody levels were comparatively lower with a single dose of either vaccine, particularly at older ages. But while the size of the immune response differed, there was no group of individuals who didn’t respond at all to either vaccine. 

There was, however, a small percentage of people – 5% – who had low responses to both vaccines, which makes it essential to monitor responses to a second vaccination. 

Antibody responses to a single dose of either vaccine also were generally lower in older individuals, especially over 60 years – unless they had had a prior COVID infection.

Antibody responses to two doses of the Pfizer-BioNTech vaccine were high across all ages, and particularly increased in older people – reaching levels similar to those who had received a single vaccine dose after a prior infection. 

David Eyre, Associate Professor at the Big Data Institute at the University of Oxford, said that while the findings still highlighted the importance of people getting a second vaccine dose degrees of protection different by age:  

“In older individuals, two vaccine doses are as effective as prior natural infection at generating antibodies to the SARS-CoV-2 virus that causes COVID-19. In younger individuals a single dose achieves the same level of response.”

Walker also emphasized the importance of vaccinations to control the spread of COVID. “Vaccines are clearly going to be the only way that we are going to have a chance to control this long term. WIthout vaccines, I don’t think getting close to zero [infections] is really feasible,” she said, during the press briefing. 

The findings come on the heels of recent  announcements by Pfizer CEO Albert Bourla that in fact a third booster jab of the vaccine may be necessary “somewhere between six and 12 months.” 

Risk of Blood Clot from AstraZeneca Vaccine Doubles, UK Still Says ‘Benefits Outweigh Risks’ 
A package of 10 multidose vials of the Oxford/AstraZeneca COVID-19 vaccine.

Meanwhile, new UK government data suggested that the risk of serious blood clots from AstraZeneca jabs was greater than had previously been reported at national level. 

The most recent survey found 168 cases of major thromboembolic events (blood clots) with concurrent thrombocytopenia (low platelet counts) were reported to the Medicines and Healthcare products Regulatory Agency (MHRA). The estimated number of first doses of AstraZeneca COVID-19 vaccines administered in the UK was 21.2 million by 14 April, giving an overall case incidence of 7.9 such events per million doses – or one person in every 125,000.  

Out of these cases, which occurred in 93 women and 75 men, 32 deaths occurred (19%). Previously, there had been 79 cases and 19 deaths reported in the UK.  

The UK government’s regulatory agency, however, continues to emphasize that the benefits still outweigh the risks. 

“On the basis of this ongoing review, the advice remains that the benefits of the vaccine outweigh the risks in the majority of people,” the MHRA said. 

Image Credits: University of Oxford, Flickr.

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