The safety and effectiveness of Pfizer and AstraZeneca COVID-19 vaccines are even better than predicted in clinical studies

Researchers in the UK and the United States have shown the effectiveness of a dose of the Pfizer-BioNTech or Oxford-AstraZeneca vaccine against severe coronavirus 2 (SARS-CoV-2) infection with acute respiratory syndrome in a real-world setting in the UNITED KINGDOM .

The novel SARS-CoV-2 virus is the causative agent of the 2019 coronavirus disease (COVID-19) pandemic, which continues to pose a threat to global public health and has now killed more than 3.1 million people worldwide.

The large prospective observational study of more than 627,000 people found that self-reported infection rates were significantly lower in participants who received either vaccine than in unvaccinated controls.

As reported in The Lancet: Infectious Diseases, documented infection rates were 58% lower after 12 to 20 days, 69% after 21 to 44 days, and 72% after 45 days after a single dose of Pfizer-BioNTech (BNT162b2) vaccine up to 59 days compared to unvaccinated controls.

After a dose of the Oxford-AstraZeneca (ChAdOx1) vaccine, documented infection rates were 39% lower after 12 to 20 days and 60% lower after 21 to 44 days than in unvaccinated controls.

In addition, the reported side effects after vaccination were moderately common, of minor severity, and of short duration, reports Cristina Menni of King’s College London and colleagues.

“Our data support resulted from randomized controlled trials in a large community-based scenario that showed evidence of a reduction in infection after 12 days and significant protection after 3 weeks,” the team writes.

The UK government decided to postpone the second dose of vaccine

In the UK, the COVID-19 vaccines Pfizer-BioNTech and Oxford-AstraZeneca began rolling out on December 8, 2020 and January 4, 2021, respectively.

At the end of December 2020, on the recommendation of the Joint Vaccination and Immunization Committee, the UK government decided to postpone the administration of the second dose.

In Phase 3 studies, efficacy was reported to be 52% twelve days after a first dose of the BNT162b2 vaccine and 95% after a second dose 3 to 4 weeks later.

However, a re-analysis of the reported efficacy data for BNT162b2 in reducing infections, serious illnesses, hospitalizations and deaths across Israel found 90% efficacy two weeks after just one dose.

A clinical study with the ChAdOx1 vaccine also reported an efficacy against symptomatic COVID-19 of 76% just 22 days after a dose.

However, according to Menni and colleagues, at this stage in the introduction of vaccination, surveillance within the UK population is needed to assess the safety and effectiveness of these vaccines in real time.

What did the study include?

The team examined self-reported side effects in 627,383 people (aged 16 to 99 years) who received one or two doses of the BNT162b2 vaccine or one dose of the ChAdOx1 vaccines between December 8, 2020 and March 10, 2021.

Side effects reported by the participants themselves in real time via the COVID Symptom Study app.

The researchers also compared the infection rates among a subset of 103,622 vaccinated people who subsequently logged at least one PCR or lateral flow test with the rates among 464,356 unvaccinated app users who also logged those test results.

What did the study find out?

Of the 627,383 people vaccinated, 282,103 received one dose of BNT162b2, of which another 28,207 received a second dose and 345,280 received a dose of ChAdOx1.

The most commonly reported systemic side effects after vaccination were fatigue and headache, which lasted an average of 1.01 days. The most commonly reported local adverse reactions were tenderness and pain at the injection site, which lasted an average of 1.02 days.

A significantly higher proportion of people who received a dose of ChAdOx1 reported systemic side effects compared to those who received a dose of BNT162b2, at 33.7% versus 13.5%.

In contrast, local side effects were reported less frequently after a dose of ChAdOx1 than after a dose of BNT162b2 (58.7% versus 71.9%)

Systemic side effects were 1 x 6 times more common after a dose of ChAdOx1 and 2 x 9 times more frequent after a dose of BNT162b2 in people with previous SARS-CoV-2 infection than in people without previous infection.

Local effects were similarly more frequent in people with a previous infection than in people without (1.4 times more often after a dose of ChAdOx1 and 1.2 times more often after a dose of BNT162b2).

What about the SARS-CoV-2 infection rate?

In the subset of participants who logged PCR or lateral flow test results, 3,106 (0.03%) of 103,622 people who were vaccinated tested positive for SARS-CoV-2 infection, compared with 50,340 (9.22%) who did 464,356 unvaccinated controls.

The team observed a significant reduction in the risk of infection from day 12 after a vaccine dose, reaching 60% for ChAdOx1 and 69% for BNT162b2 after 21 to 44 days and 72% for BNT162b2 after 45 to 59 days.

The researchers say that while the observed nature of these data does not directly reflect efficacy, the observed decrease in infection over time after one vaccine dose supports the UK government’s decision to postpone the timing of a second dose to 12 weeks, around the clock Maximize the number of people who will receive at least one dose.

However, long-term monitoring of SARS-CoV-2 protection in people who received a delayed second dose compared to people who received a second dose according to the initial guidelines (21 days after the first dose) is required to determining whether these initial protection estimates persist, the team concludes.

Journal reference:

  • Menni C et al. Vaccine Side Effects and Post-Vaccination SARS-CoV-2 Infection in COVID Symptom Study App Users in the UK: A Prospective Observational Study. The Lancet: Infectious Diseases, 2021. DOI:

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