One dose of mRNA vaccine reduced COVID risk for at least four months in health care workers

Due to a shortage in vaccines, the Quebec Immunization Committee (QIC) recommended that the Canadian province of Quebec defer the second dose of the coronavirus disease 2019 (COVID-19) vaccine to optimize the coverage of the first dose. The reason behind this proposal was to protect a majority of the high-risk population, which includes individuals in the older age groups and frontline workers, from severe COVID-19 infection and deaths.

Quebec started its vaccination campaign by targeting long-term care facility (LTCF) residents and healthcare workers (HCW) who were in direct contact with patients. Based on the vaccine supply and short-term vaccine effectiveness (VE) reports, in early March 2021, the Canadian National Advisory Committee on Immunization and the Quebec Ministry of Health set the duration between two vaccine doses to be 16 weeks.

Study: Single-dose mRNA vaccine effectiveness against SARS-CoV-2 in healthcare workers extending 16 weeks post-vaccination: a test-negative design from Quebec, Canada. Image Credit: ezps /


In Canada, two messenger ribonucleic acid (mRNA)-based vaccines, including the BNT162b2 vaccine by Pfizer-BioNTech and mRNA1273 by Moderna, received emergency use approval in December 2020. Initially, individuals were to be given their second dose of the Pfizer-BioNTech and Moderna vaccines 3 and 4 weeks after receiving the first dose, respectively.

According to the Phase III randomized-controlled trial results, the efficacy for both vaccines exceeded 90% 14 days after administration of a single dose. However, the efficacy of the vaccines beyond 3-4 weeks after the initial vaccination was not studied.

As healthcare workers (HCWs) were amongst the prioritized group to first receive the COVID-19 vaccine, many observational studies have been conducted to assess the efficacy of mRNA vaccines after a single dose. Most of these studies have a short follow-up period, with the longest being 8 weeks post-vaccination.

Efficacy of one- and two-doses of mRNA vaccines

A new study published on the preprint server medRxiv* compared the efficacy of mRNA vaccines against the original strain of the severe acute respiratory syndrome coronavirus 2 (SARS CoV-2) and SARS-CoV-2 variants of concern (VOC) among HCWs in Quebec after receiving their first and second doses. This study has also determined the effectiveness of a single dose of the mRNA vaccine at 16 weeks post-vaccination.

To do this, the researchers adopted a test-negative design that compared vaccination among HCWs who tested positive for SARS-CoV-2 to HCWs who tested negative. Taken together, 5,316 cases and 53,160 controls were included in the current study.

VE of 70% for a single dose of the mRNA vaccine against any SARS-CoV-2 infection and 73% against COVID-19 illness among the study group was reported. However, in the case of HWCs who received two doses of the vaccine, VE was calculated to be 86% for any SARS-CoV-2 infection and 93% against COVID-19 illness. VE against SARS-CoV-2 hospitalization rates for the recipients of both one and two doses was comparably high at >95%.

Vaccination coverage in the cohort of healthcare workers and total number of reported COVID-19 cases in the population per week, Quebec, Canada.

Can a single dose of COVID-19 mRNA vaccine provide enough protection?

The researchers have reported that a single dose of an mRNA vaccine provided substantial protection against SARS-CoV-2 that remained stable for at least 16 weeks post-vaccination. This is undoubtedly the biggest contribution of the paper, as previous studies have not demonstrated vaccine effectiveness beyond 8 weeks. These findings led to the conclusion that the interval between the first and second dose of the vaccine can be prolonged for up to four months in the event that there is a scarcity in the vaccine supply.

The findings described here align with similar observational studies that have been conducted using Pfizer-BioNTech vaccines in the United States, Israel, and Europe. However, these reports are based on short follow-up periods post-vaccination.

The present study has also reported that a single dose of mRNA vaccine has effectively reduced hospitalization owing to severe COVID-19 infection among HCWs by 97% across 16 weeks. This observation differs from previous studies that claim that the lowest rate of COVID-19 hospitalization is obtained after receiving two doses of the COVID-19 vaccine.

When the current study was being conducted, the Alpha (B.1.1.7) variant was the dominant circulating strain of SARS-CoV-2 in Quebec. Therefore, the authors report that the effectiveness of the single dose of the vaccine against the Alpha strain was 60% among the HCWs. This finding is consistent with prior studies conducted in another Canadian province of British Columbia, which showed a single-dose VE of 67% against COVID-19 Alpha infection in adults over the age of 70.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Priyom Bose

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.


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