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Effectiveness of mRNA boosters against symptomatic infection and severe COVID-19 in Brazil and Scotland

January 2023: Research published in PLOS Medicine looks at how well mRNA booster vaccines work in protecting people in Scotland and Brazil against COVID-19.

Effectiveness of mRNA boosters after homologous primary series with BNT162b2 or ChAdOx1 against symptomatic infection and severe COVID-19 in Brazil and Scotland: A test-negative design case–control study

Cerqueira-Silva, T; Shah, S.A; Robertson, C; Sanchez, M; et. al.


Published on: 11 January 2023

Available online at: https://doi.org/10.1371/journal.pmed.1004156

Plain English summary

In Brazil and Scotland, most adults had a ‘primary schedule’ of COVID-19 vaccines, and have since been offered booster vaccines:

  • Primary schedule: Normally two doses of Pfizer-BioNTech (BNT162b2) or Oxford-AstraZeneca (ChAdOx1) vaccine. Some higher risk groups had three vaccines as part of their primary schedule.
  • Booster vaccines: At least one dose of either Pfizer-BioNTech or Moderna (mRNA-1273) vaccine (Scotland only). Both these vaccines use ‘mRNA’ to create an immune response in the body.

At the end of 2021, the Omicron variant of coronavirus (SARS-CoV-2) emerged in Brazil, Scotland and many other places. Very quickly, it became the most common variant in both countries.

Why did we carry out this study?

How well vaccines work depends partly on the variant of the virus, and the length of time between getting vaccinated and being exposed to infection.

We know from previous research that although the Omicron variant was less likely to make people very ill compared to older variants, it also spreads much more easily than variants like Delta.

Because of this, we wanted to understand how effective booster mRNA vaccines were in protecting people against COVID-19 during the Omicron period of the pandemic, and how long this lasted.

We measured this effectiveness against both having symptoms of COVID-19, and experiencing severe COVID-19. Severe COVID-19 is defined here as being admitted to hospital or dying of COVID-19 shortly after a positive test.

What data did we use?

To do this, we used routinely collected records from 1 January to 23 April 2022, when the Omicron variant was most common. These records are secured in national databases in Brazil and Scotland.

We looked at data for people who had two doses of the same vaccine for their primary schedule, followed by an mRNA booster for COVID-19.

We calculated booster vaccine effectiveness by comparing people who had no record of a positive test with those who experienced symptomatic or severe disease. We used unvaccinated people as the reference point for measuring the effectiveness of booster vaccines over time.

The analysis used also takes account for a person’s circumstances, like age, sex, location and medical history.

What did we find?

We found that booster vaccines protected people from severe COVID-19 for about three months. Protection against having any symptoms of COVID-19 decreased much quicker.

There were 5.83 million people in our dataset; 5.28 million from Brazil and around 550,000 from Scotland.

Protection against severe COVID-19

Booster vaccine effectiveness against being hospitalised or dying from COVID-19 was high and lasted well. In Brazil, the following primary schedule vaccine followed by a Pfizer-BioNTech booster offered:

  • Oxford-AstraZeneca: A peak of 93.5%, reducing to 82.3% after four months.
  • Pfizer-BioNTech: A peak of 92.7%, reducing to 74.1% after four months.

Scotland had a similar pattern of vaccine effectiveness, peaking at 94.4 - 98.3% depending on the type of booster used. Protection dropped to 76.0 – 77.8% after four months.

Protection against symptoms of COVID-19

In Brazil, we found that the maximum booster vaccine effectiveness was 51.6%, dropping to 4.2% four months after having the vaccine. There was slightly better protection offered for people who had Oxford-AstraZeneca vaccines followed by a Pfizer-BioNTech booster, rather than Pfizer-BioNTech vaccines throughout.

In Scotland, the protection against symptomatic COVID-19 was found to be slightly better. Booster vaccine effectiveness peaked at 67.1%, before dropping to 37.4% over the same period.

Why are these results important?

In this study, we found that substantial protection against severe disease lasted for at least three months following an mRNA booster vaccine. This includes being admitted to hospital with, or dying from COVID-19.

By looking at results from two countries with different populations during a period when the Omicron period was most common, we were able to draw more reliable conclusions from our results.

This kind of information can help policy-makers, healthcare professionals and members of the public make evidence-based decisions about when to have a COVID-19 vaccine, and how frequently they should be offered.



This plain English summary was written by EAVE II's Patient and Public Involvement (PPI) Lead Dr Lana Woolford in consultation with members of the EAVE II PAG and data analysts.