6 February 2021 | New Scientist | 11PEOPLE who have been vaccinated
against covid-19 can still catch
and transmit the virus, but are less
likely to do so than unvaccinated
people, the latest results suggest.
The question of whether
vaccines halt transmission is one
of the biggest and most important
unknowns of the pandemic. If
they do, vaccine-induced herd
immunity may be possible. If they
don’t, the virus will still be able to
circulate even in a fully vaccinated
population and will continue
to pose a deadly threat to people
who haven’t been vaccinated (see
page 13) or who don’t mount an
immune response after receiving
a vaccine. Circulating virus
could also mutate and escape our
defences, reigniting the pandemic.
The latest news is mixed. “There
have been several bits of data just
in the last couple of weeks that
suggest that vaccines do not block
transmission but are very likely to
significantly reduce transmission,”
immunologist Eleanor Riley at the
University of Edinburgh, UK, said
on a Royal Society of Medicine
webinar last week.
One bit of data is from a
phase III clinical trial by vaccine
manufacturer Moderna. It found
that people given the vaccine were
a third as likely to test positive for
the virus when they returned for
their second jab as people who
got a placebo. In other words,
the first dose cuts infection rates
and hence transmission by about
66 per cent. The second shot was
given 28 days after the first and
its effect on transmission is
still unknown, because the trial
generally monitored people
after their second shot only
if they felt ill, not if they were
asymptomatically infected.
AstraZeneca and the University
of Oxford have reported some
similar figures from one of their
clinical trials. Volunteers who
Social distancing will
continue to be importantOLI^ SCARFF/GE
TTY^ IMAGES66%
Decrease in transmission rates
after a dose of Moderna’s vaccine1/
Reduction in asymptomatic
infections in people over 60,
two weeks after a Pfizer/
BioNTech vaccine dose4%
Reduction in asymptomatic
infections after two full doses
of the Oxford/AstraZeneca vaccineGraham Lawton
How well do vaccines stop people
catching and spreading coronavirus?
Transmission
received the half dose/full dose
regime – which was discovered by
accident – were 60 per cent less
likely to have an asymptomatic
infection than people who got a
placebo. However, people given the
full doses were just 4 per cent less
likely to get such infection than
people who received a placebo.
A study in China found
that infected people without
symptoms are much less likely
than those with symptoms to
transmit the virus to others in
their household, which suggests
that even if vaccines don’t
prevent asymptomatic infections,
they can still cut the transmission
rate considerably.
Finally, a study by the Clalit
Research Institute in Israel found
that the vaccine developed by
Pfizer and BioNTech reduced
asymptomatic infections in
people over 60 by about a third.
The effect didn’t kick in until
14 days after the first dose.
Taken together, these results
strongly suggest that vaccines cansignificantly, but not entirely, halt
the spread of the virus, says Riley.
“There are no data per se
on infectiousness of vaccine
recipients – the link is indirect,”
she says. “Both symptomatic
and asymptomatic infectionare less likely after vaccination
and asymptomatic people less
likely to transmit.”
This means that we will have to
continue measures such as social
distancing to slow transmission
until everybody has been fully
vaccinated. The Clalit Research
Institute warned that its results
“clearly indicate that there is
no complete protection against
corona infection... Therefore,
even those who have been
vaccinated must wear masks
in public space and observe
the rules of social distance.” ❚“ Even those who have been
vaccinated must wear
masks in public and
observe social distancing”