Researchers in Singapore have conducted a study showing that the messenger RNA- (mRNA) based coronavirus disease 2019 (COVID-19) vaccines developed by Pfizer-BioNTech and Moderna are highly effective at protecting against symptomatic and severe disease following infection with the rapidly spreading B.1.617.2 (delta) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

The team conducted a multi-center retrospective cohort study of vaccinated and unvaccinated individuals who had been admitted to hospital following infection with the B.1.617.2 variant of concern.

“To our knowledge, we provide the first data characterizing the impact of vaccination on virologic kinetics by the B.1.617.2 variant,” writes Barnaby Young from the National Centre for Infectious Diseases in Singapore and colleagues.

The team found that individuals fully vaccinated with either Pfizer-BioNTech’s BNT162b2 vaccine or Moderna’s mRNA-1273 product were significantly less likely to develop moderate or severe outcomes following B.1.617.2 infection than unvaccinated individuals.

Vaccination was associated with fewer symptoms, lower peak biomarkers of systemic inflammation, and better clinical outcomes. It was also associated with a more rapid decline in viral RNA load and a robust serologic response.

“Vaccination remains a key strategy for control of COVID-19 pandemic,” says Young and colleagues.

A pre-print version of the research paper isa available on the medRxiv* server, while the article undergoes peer review.

Variants of concern pose a threat to vaccination efforts

Phase 3 clinical trials of mRNA-based vaccines have demonstrated efficacies of 92% to 95% in preventing symptomatic and severe disease following SARS-CoV-2 infection.

While the mass rollout of these vaccines has reduced infection and mortality rates in many countries, the emergence of SARS-CoV-2 variants containing mutations in the viral spike protein has led to growing concerns regarding increased transmissibility and resistance to vaccine-induced immunity.

The spike protein is the main structure the virus uses to infect cells and a primary target of binding and neutralizing antibodies following natural infection or vaccination.

While variants of concern such as B.1.1.7 (alpha), B.1.351 (beta), P.1 (gamma), and B.1.617.2 (delta) have all been shown to exhibit increased transmissibility, the B.1.1.7 and B.1.617.2 strains have also been associated with increased disease severity and hospitalization.

Following the emergence of B.1.617.2 in India, this variant rapidly spread to other countries and had become the most frequently sequenced lineage worldwide by the end of June 2021.

The vaccination program in Singapore

The COVID-19 vaccination program began in Singapore on December 30^th, 2020. Free vaccination with either the Pfizer-BioNTech or Moderna product was made available to all Singapore residents, beginning with the elderly and those with high-risk occupations such as healthcare workers.

As of July 19^th, 2021, more than 6,837,000 vaccine doses had been administered and approximately 2,792,400 individuals had been fully vaccinated.

What did the researchers do?

Young and colleagues conducted a multi-center retrospective cohort study to characterize the clinical, virologic and serologic features of vaccinated adults with breakthrough B.1.617.2 infection. The results were compared with those of unvaccinated patients who also had B.1.617.2 infection.

Participants were recruited between April 1^st and June 14^th, 2021, across five study sites: the National Centre for Infectious Diseases, Singapore General Hospital, National University Hospital, Changi General Hospital and Sengkang Hospital.