Population-level cohort study evaluates efficacy and safety of all COVID-19 vaccines used in Denmark

By Neha MathurReviewed by Aimee MolineuxOct 13 2022

In a recent study posted to the medRxiv* preprint server, researchers investigated the long-term efficacy, safety, and durability of all severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines used in Denmark.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Background

Denmark plans to administer the fourth dose of coronavirus disease 2019 (COVID-19) vaccines to selected population segments. Further, the Danish government has plans to introduce Omicron-specific bivalent vaccines in their vaccination programs. Thus, there is an ongoing need to monitor long-term vaccine efficacy (VE) and safety, especially in high-risk groups. Notably, VE, magnitude, and durability of response to different vaccines changes against emerging SARS-CoV-2 variants of concern (VOCs), especially Omicron and its sub-lineages.

About the study

In the present study, researchers enrolled 6943 adult participants scheduled to receive a SARS-CoV-2 vaccine through the Danish COVID-19 vaccination program and called them the ENFORCE cohort. They arranged five pre-specified follow-up visits with these participants after they received their first vaccination during the follow-up period of two years. Further, the team collected blood samples for serological analysis after each study visit.

The enrollment for the ENFORCE study began 14 days up to 30 minutes before the first SARS-CoV-2 vaccine dose was administered at vaccination centers near seven study sites or hospital clinics across five Danish regions. The team invited potential study participants to join the study through several channels; for instance, they invited healthcare workers (HCWs) through hospital information channels. The study enrollment ran from 13 February 2021 to 5 August 2021.

Additionally, the researchers identified the risk factors for humoral hypo-responsiveness and assessed the durability of the vaccine response. Furthermore, the team compared the risk of breakthrough infection after the emergence of Omicron. They stratified at the levels of SARS-CoV-2 spike (S) immunoglobulin G (IgG) by the SARS-CoV-2 variant.

Study findings

The ENFORCE analysis covered 6943 participants with a median age of 64 years. Nearly 57% of the study population was female, and 23% of the study cohort had an increased risk of severe COVID-19 due to concomitant diseases. The adenoviral vector/mRNA vaccine group comprised 499 relatively young participants with a median age of 45. Of these 499 participants, 90.8% were HCWs, and a higher proportion had a positive SARS-CoV-2 reverse transcriptase-polymerase chain reaction (RT-PCR) before baseline. There were 3824 participants in the Pfizer-BioNTech vaccine group, of which 37.6% were at an increased risk of severe illness. These participants were generally older, with a median age of 71 years.

Nearly 5% of the total 6889 participants tested positive for SARS-CoV-2 spike (S) immunoglobulin G (IgG) at baseline, indicating a previous infection with SARS-CoV-2. Of these, 10.8% belonged to the age group of 18 to 25 years, while 2.9% were over the age of 80 years. The authors observed that hyporesponsiveness to the vaccine and non-durable vaccine response correlated with male sex, comorbidities, and vaccine type.

Further, the researchers found an inverse association between increasing levels of S IgG and the risk of breakthrough infection by the Delta variant. On the contrary, S IgG levels did not increase the risk of Omicron breakthrough infection. Most importantly, neutralizing capacity of S IgG decreased markedly against each emergent variant compared to the wild-type SARS-CoV-2 strain following vaccination. This decrease was most prominent against Omicron. However, it again increased after the receipt of a booster dose.

Conclusions

The ENFORCE study remarkably measured SARS-CoV-2 specific humoral and cellular immunological response. It repeatedly evaluated the safety data and microbiological test results of all the recipients for almost two years. The study cohort had many elderly and people with significant comorbidities. These individuals are of particular interest when evaluating vaccine effectiveness and durability because they are at high risk of developing severe COVID-19. The preliminary study results are available to the public on the ENFORCE website. The participants could even subscribe to receive regular updates on the study's progress, including their serology results.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources