An experimental vaccine may help protect
against a coronavirus infection, preliminary results from people and mice
suggest.
One or two doses of an mRNA vaccine prod
people’s bodies to make
as many or more antibodies against the coronavirus as are made by people
who have recovered from COVID-19, researchers from Moderna, Inc., announced May
18.
Moderna, based in Cambridge, Mass., and
the U.S. National Institute of Allergy and Infectious Diseases in Bethesda,
Md., worked
together to develop the vaccine, known as mRNA-1273 (SN: 2/21/20).
Their approach uses messenger RNA, or
mRNA, a genetic molecule that cellular machinery reads in order to build
proteins. In this case, the mRNA contains instructions for building the
coronavirus’ spike protein, which helps the virus enter human cells. The
vaccine induces human cells to make the spike protein. The immune system then makes
antibodies to latch onto the spike proteins. Should a vaccinated person
encounter the virus later, those vaccine-stimulated antibodies may prevent the
virus from infecting healthy cells.
Mice vaccinated with these mRNAs were
protected against lung infection when researchers later exposed the rodents to SARS-CoV-2,
the coronavirus that causes COVID-19, the researchers also report. None of the
data has been reviewed by independent scientists, and has not yet been made
publicly available.
The new data “give us confidence that
mRNA-1273 has a high probability of providing protection against COVID-19
disease in humans,” said Stéphane Bancel, chief executive officer at Moderna.
Researchers in Seattle and Atlanta began
injecting
human volunteers with the experimental vaccine in March (SN: 4/10/20). This first phase of
testing was designed to determine safety, help researchers determine what dose
to use, and to measure people’s immune response.
The study tested three doses: 25
micrograms, 100 micrograms and 250 micrograms. People in each dosage group got
an initial injection, followed a month later by a booster shot. Although 45
people participated in the study (15 in each group), antibody results for only the
first four people in the 25-microgram and 100-microgram groups are available.
Some preliminary results for people in the 250-microgram group were also
reported.
Levels of antibodies called neutralizing
antibodies, which can block the coronavirus from infecting human cells,
measured two weeks after the booster shot were equivalent between the 25-microgram
vaccine group and in people who had recovered from a COVID-19 infection. Levels
of neutralizing antibodies in the blood of the 100-microgram group were even
higher than in people who had recovered from the disease.
“These data are extremely promising, but they are
in eight people,” says Brianne Barker, an immunologist at Drew University in
Madison, N.J. “I agree with Moderna and NIAID that they should keep going” with
the vaccine, she says. “But I don’t know that we fully know yet that this is
going to be a home run.”
Other scientists also say that the available data
are too minimal to draw firm conclusions. Neutralizing antibody levels in blood
can vary widely, says Paul Bieniasz, a virologist at the Rockefeller University
in New York City. “So when Moderna says ‘levels matching or exceeding the level
found in [recovered people],’ that’s almost meaningless.” And no one yet knows
what level of antibody production is necessary for protection in people, nor
how long such protection might last.
The study measured only one type of immunity, that
produced by neutralizing antibodies. The researchers didn’t measure whether
other immune cells called T cells also rev up virus defenses in response to the
vaccine, Barker says. Such T cells may protect
people from developing severe cases of COVID-19 (SN: 5/15/20).
One person in the 100-microgram group
developed redness at the injection site, and three people in the 250-microgram
group developed some severe flulike symptoms, including muscle aches, headache
and fatigue, after the second injection. Those symptoms are probably
indications that the immune system is responding to the vaccine, said Tal Zaks,
Moderna’s chief medical officer, during a May 18 briefing with reporters and
investors.
But those symptoms could be cause for
concern. The injection site redness was classified as severe, or grade 3.
That’s a red spot 10 centimeters in diameter, says Mark Slifka, a viral
immunologist at Oregon Health & Science University in Portland. The next
step is death of the tissue. Milder side effects are typically common with
vaccines, “but if there are a consistent number of grade 3s, that’s a concern
for a vaccine you want to use broadly.”
The company has already gotten
permission from the U.S. Food and Drug Administration to go ahead with the
second phase of human safety tests. The company will eliminate the
250-microgram-dose arm and add a 50-microgram group. Lower doses may offer
equal protection against the virus and stretch limited supplies of the vaccine,
Zaks said.
The initial tests were in people 18 to
55 years old. The next phase of testing will also include people 56 to 70 years
old, and an over-70 group.
A much larger third phase of testing,
which would continue safety testing and measure antibody responses, will also
determine whether the antibodies actually protect against infection. That phase
may begin as early
as July.
