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By Kate Kelland
Nov 27 (Reuters) - The frontrunners in the COVID-19 vaccine
race have emerged with different success rates for their shots
in clinical trials, but what does that mean for the global fight
against the pandemic?
U.S. drugmakers Pfizer-BioNtech and Moderna
have each said their coronavirus vaccines have an
efficacy rate of around 95% and a Russian project touted 92%
efficacy for its Sputnik V vaccine.
Britain's AstraZeneca announced an average efficacy rate of
70%, still well above the 50% rate that U.S. regulators have
said they want to see before approving a COVID-19 vaccine for
use.
WHAT ARE EFFICACY NUMBERS?
If a vaccine has an efficacy of, say, 80%, it means that if
100 people who have not previously been infected by the
coronavirus are given the vaccine, on average 80 of them will
not get the disease that the virus causes: COVID-19. These rates
relate to vaccines administered and monitored in controlled
circumstances, such as clinical trials.
DOES THAT MEAN AN INDIVIDUAL'S PROTECTION LEVEL IS THE SAME?
No. A person immunised with a vaccine that has, say, 80%
efficacy is very likely to be protected from getting the disease
with symptoms, especially severe ones.
They are also very likely to be protected from asymptomatic
disease - but this, depending on the vaccine, may be less
certain.
Even with 95% efficacy, there is no absolute guarantee of
protection for any particular individual.
WHAT DOES THIS MEAN IN THE REAL WORLD?
There is a difference between efficacy rates obtained in
clinical trials and effectiveness - the real-world protection
rate of a vaccine when it is rolled out.
"Efficacy says: 'Does it work?'. Effectiveness says: 'Can it
be applied? Can you carry the efficacy to the people?'," said
Marcel Tanner, an epidemiologist and president of
Switzerland's Academies of Arts and Sciences.
In the real world, a vaccine's effectiveness can be
influenced by multiple, unpredictable factors including, for
example: the rate of spread of a virus; how many, or few, people
adhere to the optimum dosing schedule and timetable; how
individuals' immune systems respond; whether the vaccine was
stored at the correct temperature; whether people know, or don't
know, if they've been exposed before.
Generally, a vaccine's real-world effectiveness tends to be
slightly lower than its efficacy.
WILL THESE VACCINES STOP THE COVID-19 PANDEMIC?
Experts say this is unlikely. More realistic, they say, is
that we will have to live alongside the SARS-CoV-2 virus.
Evidence so far suggests that COVID-19 vaccines developed by
Pfizer-BioNtech, Moderna and AstraZeneca will help stop people
developing the disease. Only AstraZeneca's data, so far, shows
signs that its shot may also help prevent transmission of the
virus.
"Protection against illness has a value for an individual,"
said Penny Ward," a visiting professor in pharmaceutical
medicine at King’s College London. She added, however, that
vaccines that do not prevent transmission will not halt the
pandemic.
"Until the vaccination and other measures result in the
virus being close to elimination in any particular country, and
worldwide, there will still be a need for distancing, masks and
hand washing to reduce transmission further than will be
achieved by the vaccine alone," said Stephen Evans, a professor
of pharmacoepidemiology at the London School of Hygiene and
Tropical Medicine.
"(A) vaccine is no good until people are vaccinated, and
even then, it will not result in a situation where all other
protective measures can be immediately abandoned."
(Reporting by Kate Kelland in London and John Miller in Zurich;
Editing by Kevin Liffey)
