By Lisa Marshall at University of Colorado, Boulder
Dec. 10, 2020 -- Should people who
already had COVID-19 step aside and give their place in the vaccine line to
someone else? In some cases, yes, suggests new
University of Colorado Boulder research.
“Our research suggests that prioritizing
people who have not yet had COVID could allow hard-hit communities to stretch
those first doses farther and get to some of the herd immunity effects sooner,”
said Dan Larremore, a computational biologist at the BioFrontiers Institute
whose team used mathematical modeling to determine how different distribution
strategies could play out in cities around the globe.
He and lead author Kate Bubar, a
graduate student in the Department of Applied Mathematics, teamed up with
colleagues at the Harvard T.H. Chan School of Public Health and the University
of Chicago, to do the study. They found that if, in general, saving lives is
the objective, enabling people 60 and older to go to the front of the line
works best. If reducing future infections is the goal, however, the 20- to
49-year-old crowd should get the first doses. In very few cases did vaccinating
children first make sense.
For the study the researchers used
demographic data — including age distribution and published contact rates amid
and between different age groups — from countries around the world.
They also drew on information on the percentage
of people who are already “seropositive” or previously infected with the virus
in different locations.
Then they modeled what would happen in
five different scenarios in which a different group got to be first in line:
Children and teenagers, adults between the ages of 20 and 49, adults aged 20 or
older or adults aged 60 or older. In the fifth scenario, there was no
distribution strategy and anyone who wanted a vaccine got one while supplies
lasted.
Results from the United States, Belgium,
Brazil, China, India, Poland, South Africa and Spain are included in the
paper.
Local circumstances matter
Different strategies worked better or
worse, depending on each area’s population but a few findings jumped out.
“In the broadest array of scenarios,
across countries, prioritizing adults 60-plus first was the best way to
minimize mortality,” said Larremore. “If we want to go back to pre-pandemic
behavior, giving the first wave of vaccines after healthcare workers to older
adults is the way to go.”
That finding has already been used by
global health experts to inform vaccine distribution strategies, with the World
Health Organization and the Centers for Disease Control referencing the study
in recommendations to prioritize older adults.
But after that, as individual states
determine who’s next, the story gets more complicated.
“If transmission is rampant, and
hospitals are being overwhelmed, then directly protecting those who are at the
highest risk for severe outcomes is the best way to save lives and decrease the
stress on our healthcare system,” said Bubar. “But, if instead, transmission is
relatively low in a given area then prioritizing those who have the most
contacts would be better— provided that the vaccine blocks transmission.”
In communities where COVID had already
infected large swaths of the population, prioritizing those who are
“seronegative” or did not already test positive for the virus, could allow
health agencies to stretch the vaccine farther and save more lives.
For instance, in New York City where 27%
of people have already been infected, vaccinating one in five people over age
60 could bring mortality down by 73%.
“But the city could get that same level of population protection by vaccinating
just one in six older adults —if those without antibodies were brought to the
front of the line,” said Larremore.
A Covid test before your shot?
Just how might a city go about
identifying people who’d already been infected?
In cases where vaccine availability was
scarce, the authors said, it might be worth having individuals take an antibody
test before getting the vaccine. Alternatively, people who’d already had
COVID-19 could just be asked to consider sparing their dose for someone who
does not already have partial protection from the virus.
In a city like Boulder, however, where
only a small percentage of people have already been exposed, testing people
prior to vaccination might not yield enough benefit to be worth the effort.
“We hope that state governments will
consider the local status of the epidemic in their decisions about who to
prioritize,” said Bubar.
This work is under peer review, and the
authors hope to publish in early 2021. Meantime, as vaccines become available
and policymakers begin to make tough decisions influenced by emotions, ethics
and the economy, the authors hope their work can provide some statistical
footing.
“We need policy decisions that are
well-anchored in the most rigorous scientific estimates,” said Larremore. “This
paper is all about providing them.”
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