Canada has so far approved four different coronavirus vaccines.
During the global clinical trials, the shots from Pfizer, Moderna,
AstraZeneca and Johnson & Johnson showed high efficacy rates –
ranging from 62 per cent to 95 per cent – in preventing moderate to
severe COVID-19 illness.
But there are still many unanswered questions about whether
the vaccines can also suppress transmission – that is, keep vaccinated
people from acquiring the virus and spreading it to others.
are a number of ongoing studies looking at the effectiveness of the
vaccines on transmission – and preliminary data is promising.
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According to a pre-print analysis
in The Lancet medical journal – not yet peer-reviewed – the
AstraZeneca-Oxford vaccine was shown to reduce the rate of positive PCR
tests by half when two doses were given. There was a 67 per cent
reduction after one dose.
A study by Cambridge University in the U.K. suggests that a single dose of the Pfizer-BioNTech vaccine can reduce fourfold the number of asymptomatic COVID-19 infections. These results released on Feb. 26 are yet to be peer-reviewed.
Meanwhile, another Israeli study
published in the Lancet medical journal
on Feb. 18 found an 85 per cent reduction in symptomatic COVID-19
infections within 15 to 28 days of receiving a first dose of the Pfizer
vaccine, with an overall reduction in infections, including asymptomatic
cases detected by testing, of 75 per cent.
These are encouraging signs for ending the pandemic, as preventing
infection in those who have been vaccinated means there is no virus that
can be passed on to others.
“If you never get infected,
then it’s also possible that you won’t transmit it to other people,”
said Gerald Evans, an infectious disease specialist at Queen’s
University in Kingston, Ont.
Pfizer is currently testing a subset
of Phase 3 trial participants to look for evidence of symptomatic and
“Understanding whether the vaccine can stop the transmission of the
virus will be an important tool for the continued rollout of the vaccine
and to inform public health planning efforts and recommendations,”
Christina Antoniou, director of corporate affairs of Pfizer Canada, told
Global News in an emailed statement.
According to the World Health Organization (WHO)
, there is limited evidence about the performance of the COVID-19 vaccines to prevent infection or transmission.
it is “reasonable to assume there will be some level of protection
against transmission,” the U.N. health agency said in an emailed
statement to Global News.
Evans said while more data is needed to
determine the vaccination’s impact in real-world settings, he believes
vaccines are the “number one tool” at our disposal and will
substantially curb COVID-19 spread.
“I’m estimating about 75 to 80 per cent reduction in transmission,” he said.