EAVE II
EAVE II logo

Vaccine Effectiveness of Two-Dose BNT162b2 Over Time Against COVID-19 Symptomatic Infection and Severe Cases Among Adolescents

April 2022 (Pre-print): Research published in The Lancet looks at the effectiveness of two COVID-19 vaccine doses in adolescents over time.

Vaccine Effectiveness of Two-Dose BNT162b2 Over Time Against COVID-19 Symptomatic Infection and Severe Cases Among Adolescents: Test Negative Design Case Control Studies in Brazil and Scotland

Florentino, P.T.V; Millington, T; Cerqueira-Silva, T; Robertson, C; et. al.

The Lancet

Published on: 5 April 2022

Available via: http://dx.doi.org/10.2139/ssrn.4074678

Plain English Summary

The Pfizer-BioNTech vaccine has been shown to be safe and effective in adolescents aged 12-17 years, following clinical trials and smaller observational studies. Many countries, including the UK and Brazil, started to roll out vaccines for this age group in the second half of 2021.

Why did we carry out this work?

Currently, little is known about how COVID-19 vaccine effectiveness (VE) changes over time in teenagers. This is especially the case with the Omicron variant of coronavirus (SARS-CoV-2), known as B1.1.529.

To date, nobody has reported data about how well teenagers are protected against infection and disease after two doses of a COVID-19 vaccine.

Understanding how vaccine protection wanes is important because it can help to inform future vaccine programmes for young people, and potentially prevent further illness and education disruption.

In this study, we looked at symptomatic SARS-CoV-2 infection and severe COVID-19 in teenagers in Scotland and Brazil. We studied different time periods after receiving two doses of the Pfizer BioNTech vaccine (BNT162b2).

What data did we use?

We looked at the data of adolescents aged 12-17 years old with COVID-19 related symptoms for the time periods of 8 September 2021 – 8 March 2022 in Brazil, and 6 August 2021 – 1 March 2022 in Scotland.

We did this by linking records of RT-PCR (‘Reverse Transcription-Polymerase Chain Reaction’) and antigen tests for SARS-CoV-2 to national vaccination and clinical records in both countries.

Find out more about: What is PCR?

In the models for both countries we accounted for people’s individual circumstances including:

  • Age
  • Gender
  • Time (week) a person tested positive for infection
  • Location
  • Previous SARS-CoV-2 infection
  • Health conditions that put people at higher COVID-19 risk 

In Brazil, we also considered ethnicity, current pregnancy or being in the post-partum period (usually 6-8 weeks after delivery). Unfortunately, we did not have enough data available to consider these factors in our Scottish analysis.

Data from Brazil

To create our Brazilian cohort, we used data about RT-PCR and antigen testing, vaccinations, as well as registered COVID-19 related hospital admissions and deaths.

These data sources were linked using information provided by DATASUS, the Health Data Informatics department at the Brazilian Ministry for Health.

Data from Scotland

We gathered our Scottish data through the Early Pandemic Evaluation and Enhanced Surveillance (EAVE II) project, which has been able to link multiple, de-personalised datasets for almost all the Scottish population, in near-real time.

Find out more about EAVE II

What did we find?

In total, we analysed 447,882 tests from Brazil and 375,385 tests from Scotland.

Vaccine effectiveness against Delta and Omicron Variants

We found that vaccine effectiveness against symptomatic COVID-19 varied depending on which variant was the most dominant at a given point in time. Vaccines were less likely to protect people during the Omicron period compared to the Delta period.

This was consistent across all time intervals that we examined in both countries: up to 14 days post-first dose, up to 14 days post-second dose and every two-week period thereafter until over 98 days post-second dose.

We found that peak vaccine protection occurred 14-27 days after the second vaccine dose, for both countries and both variants. Vaccine effectiveness was higher in the Delta period than the Omicron period:

  • Delta period: 85.8% in Brazil and 90.7% in Scotland
  • Omicron period: 62.8% in Brazil and 78.3% in Scotland.

Throughout the entire Delta period, VE in Scotland was consistently higher than that in Brazil.  

Vaccine effectiveness against severe disease over time

In both Scotland and Brazil, we found that VE began to decline after 27-days post-second dose.

Our results also showed that, after 98 days, the effectiveness of two vaccine doses fell to 31.3% in Scotland, and 13.9% in Brazil.  

However, after 98 days, vaccine effectiveness against severe disease remained very high, at over 80% in Brazil. An equivalent result could not be made for Scotland due to the small number of severe COVID-19 related cases that occurred overall.

The difference in results for effectiveness against infection and serious disease is most likely to be related to different parts of a person’s immune response, which includes both antibodies and immune cells (e.g. T cells). The protection of antibodies wanes over time, which increases the risk of infection. However, a person’s T-cell response also protects and limits the progression of COVID-19, and can prevent serious disease.

Why is this important?

To our knowledge, this is the first multi-nationwide study to evaluate how vaccine effectiveness against severe COVID-19 waned for 12–17-year-olds during the Omicron period. Our results showed that, during the Omicron period, the protection given by the Pfizer- BioNTech vaccine against severe COVID-19 remained strong 98 days after the second dose.

However, vaccine protection against symptomatic infection began to wane 27-days post-second dose.

This suggests that two -doses are not enough to continue stopping the spread of the SARS-CoV-2 virus, meaning it is unlikely to prevent further school disruptions, or protect against COVID-19 infection in the long term.

We therefore support the rollout of third and booster dose vaccinations to this age group.