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Early findings show the first vaccine dose reduced the risk of COVID-19 by 80 per cent or more

Preliminary results from the BCCDC show a single dose of COVID-19 mRNA vaccine is already giving substantial protection to B.C. residents of long term care facilities and health care workers.
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​Early vaccine effectiveness results from British Columbia (B.C.) show the first dose of COVID-19 vaccine reduced the risk of COVID-19 in long term care residents and health care workers by 80 per cent within two to three weeks of receiving the vaccine. 

“These findings, based on surveillance data, are very promising and reinforce the substantial benefit provided by the first dose of the mRNA COVID-19 vaccine in these priority populations,” said Dr. Danuta Skowronski, lead for the Influenza and Emerging Respiratory Pathogens Team at the BC Centre for Disease Control (BCCDC). “They also help to answer one of the important unanswered questions after the clinical trials about the effectiveness of the vaccines in the elderly and notably those within long term care.”

The analysis, led by Dr. Skowronski, looked at COVID-19 cases that occurred among vaccinated long term care residents and health care workers between late December 2020 and early February 2021. Researchers observed a pronounced drop in the number of cases among vaccinated individuals in both groups, beginning about 14 days after vaccination, including a reduction in hospitalizations and deaths among vaccinated long term care residents.

Vaccines begin to work a couple of weeks after vaccination. Researchers compared the risk of becoming a COVID-19 case in the first ten days following vaccination, before the vaccine would have had an effect, to the risk two weeks or more after vaccination.  In that way they were able to show vaccine effectiveness of 80 per cent or more beginning just a few weeks after vaccination. This means that a single dose of vaccine could prevent at least eight out of every 10 cases of COVID-19. 

The findings are encouraging as about 60 per cent of deaths in B.C. have occurred in residents of long-term care facilities. Since the COVID-19 vaccination program began, there has been a demonstrable decline in outbreaks and deaths in long-term care facilities. 

In B.C., residents and staff of long term care and assisted living residences and front-line health care workers were prioritized in Phase 1 of the COVID-19 Immunization Plan to protect those most at risk of death and severe outcomes. Vaccinations for both groups began in December 2020.

The results are based on surveillance observations and have not yet been published in a peer reviewed paper but are being shared in the context of a public health emergency. They are comparable to vaccine effectiveness results released by Quebec. Together, the studies reinforce the substantial protection for these important populations afforded by a single dose of COVID-19 vaccine.

Funding for this research was provided by the Michael Smith Foundation for Health Research.

ThBC Centre for Disease Control, a part of the Provincial Health Services Authority, provides public health leadership through surveillance, detection, treatment, prevention and consultation services. The Centre provides diagnostic and treatment services for people with diseases of public health importance, and analytical and policy support to all levels of government and health authorities. The BCCDC also provides health promotion and prevention services to reduce the burden of chronic disease,  preventable injury and environmental health risks. For more, visit  or follow us on Twitter @CDCofBC.

The Provincial Health Services Authority plans, manages and evaluates selected specialty health care services across BC, working with the five regional health authorities, First Nations Health Authority and the Ministry of Health to deliver province-wide solutions that improve the health of British Columbians. For more information, visit or follow us @PHSAofBC.


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