May 5, 20211:05 PM ET JOE
PALCA
Typically,
if you get a COVID-19 vaccine that requires two doses, you should get two of
the same vaccine. Two Pfizer shots, or two Moderna shots. Not one and then the
other.
But in
the future, that could change, either by necessity or by design.
This
idea of using two types of vaccines isn't a new concept. It's known as
heterologous vaccination, although there's a more colloquial term.
"In
the U.K. at the moment, we're sort of calling it 'mix and match,' "
says Helen Fletcher, a professor of immunology at the London
School of Hygiene & Tropical Medicine. She says shortages of a vaccine or
concerns about side effects may induce health officials to adopt a mix-and-match
strategy.
Health
agencies in France and Germany are already encouraging people who've gotten the
AstraZeneca vaccine to consider getting one of the mRNA vaccines for their second shot.
"So
there's a practical reason why you would want to mix two different types of
vaccine. But there is also a scientific reason as well," Fletcher says.
Basically,
all vaccines work by showing people's immune systems something that looks like
an invading virus but really isn't. If the real virus ever comes along, their
immune systems will recognize it and be prepared to fight it off.
Using
two different vaccines is a bit like giving the immune system two pictures of
the virus, maybe one face-on and one in profile.
"If
you give two different types of vaccine, then you tend to get a better immune
response than if you give the same vaccine twice," Fletcher says.
Some
vaccine manufacturers have embraced this approach and are making vaccines of
two different types by design.
One is
a company called Gritstone bio, based in Emeryville, Calif.
"The
natural human response to a virus is to mobilize two distinct arms of the
immune system," says Gritstone CEO Andrew
Allen. One utilizes antibodies; the other relies on something
called CD8 T cells. Unlike antibodies, CD8 T cells don't
recognize a virus directly, but they do recognize a cell that has been infected
by a virus and they can destroy the infected cell.
Gritstone
has developed two different vaccines to activate each arm: a viral vector vaccine and an mRNA vaccine. The
viral vector vaccine is very good at stimulating the production of CD8 T cells.
"The
mRNA [vaccine] makes a really good antibody response. And so potentially by
combining these, you kind of get the best of both worlds," Allen says.
Gritstone's
approach is already being tested in human volunteers.
This
mix-and-match approach has been tried with vaccines for a variety of diseases.
"Diseases
such as HIV, malaria, TB, even influenza," says Bali
Pulendran, Violetta L. Horton professor and professor of immunology
and microbiology at Stanford University. "So there's ample evidence for
the benefits of such strategies."
If it's
such a good idea, why isn't it used routinely?
Pulendran
speculates there are two main reasons. One has to do with the way new vaccines
are approved.
"Regulatory
authorities love simplicity," Pulendran says. "The simpler the
vaccine regimen, the more palatable they find this to be."
The
other reason is that while mix and match may make scientific sense, it doesn't
always make business sense.
"For
example, if Company A makes one vaccine and Company B makes another vaccine,
unless there's some overarching incentive for the two companies to enter into
some sort of a marriage, I think either company would in general prefer to go
along with their own," Pulendran says.
Of
course, if a small company like Gritstone shows that a mix-and-match strategy
really leads to a dramatically better vaccine, you can bet other pharmaceutical
companies will find a way to solve the business problem.
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