Three of the four vaccines
approved by the European Medicines Agency (Pfizer/BioNTech, Moderna and
AstraZeneca) require two injections.
So, what is the purpose of the second dose?
The first dose introduces
the target antigen into the body, while the second dose reinforces its action
and prolongs the duration of immune response.
This is especially true
for messenger RNA vaccines.
DR William Petri,
professor of medicine at the University of Virginia says about Covid vaccine:
"These vaccines need
a booster to reactivate T cells, which in turn stimulate antibody-producing
memory B cells."
The second dose amplifies
the activation of immune system cells and leads to a more intense and prolonged
response.
Interval between the first and second dose
These are purely empirical
data," admits a spokeswoman for Professor Alain Fischer, who is in charge
of vaccine strategy in France.
In other words, they are
recommended by the laboratories themselves according to clinical trials and are
not necessarily to be taken literally.
In January, France decided
to postpone the second injection of the Pfizer vaccine to 42 days instead of
the 21 days initially recommended.
In the UK, the second injection is even
delayed until 12 weeks after the first dose.
For the AstraZeneca
vaccine, a study in The Lancet showed that the efficacy of the vaccine was
higher with a 12-week interval than with a 6-week interval.
Efficacy of 81% VS. 55% with
a twice as high antibody binding response.
In short, extending the
time between doses does not seem to be a problem.
Above all, this strategy
makes it possible to vaccinate a maximum number of people more quickly without
having to set aside doses.
Could we do without a second dose?
The difference between the
two doses is not necessarily obvious.
The National Agency for
the Safety of Medicines (ANSM) reports an efficacy of 86.7% 10 days after the
first injection for the Pfizer vaccine.
While, the efficacy was of
95% in 7 days after the second dose.
So why try to vaccinate
twice at all costs? "The risk is that of a lesser efficacy that is much
less long-lasting," warns Professor Fischer's team.
"The duration of clinical
protection achieved after a first dose is not known.
And is not guaranteed
beyond the administration window retained in the study protocol" (i.e. 42
days), reminds the ANSM. "
Administering a single
dose of vaccine is not an option whose efficacy has been established,
particularly over time.
It is therefore not an
option that can be considered deliberately or even less systematically."
Johnson & Johnson vaccine only require 1 dose
The Janssen vaccine (a
subsidiary of Johnson & Johnson) validated by the European Medicines Agency
on March 12.
It promises 66% efficacy
after a single dose and even 76% against severe forms of the disease.
But for the laboratory,
this is more of a promotional argument than a real scientific difference.
Because the participants
were followed for only 8 weeks after vaccination, and there is no indication
that the immune response will not decrease afterwards.
In fact, the laboratory
began trials in November 2020 with two injections spaced 57 days apart.
According to Inserm, which
is overseeing the study:
"Preliminary data
show that the two-dose vaccine regimen increases the amount of antibodies
produced against SARS-CoV-2 by a factor of 2 to 3."
The nature of this vaccine
is an adenovirus that serves as a genetic vector for SARS-CoV-2 proteins.
That is means that a second injection would not
necessarily be beneficial.
The immune system could
recognize the vector virus during the second dose and eliminate it before it
can act.
A single dose for people already infected
In people already infected
by the virus, it is however established that only one dose of vaccine is
necessary.
"People who have had
an infection with SARS-CoV-2 should be considered protected for at least 3
months by post-infectious immunity [...],"
That is whether or not
they have developed a symptomatic from Covid-19, says the HAS.
"The single dose of
vaccine will then play a booster role."
In this case, the
vaccination must be performed within 6 months after the infection.
Millions of Americans miss second dose
About 8 percent of people
who received a first injection from Pfizer or Moderna were never given a second
one.
After the distrust of
Covid-19 vaccines, U.S. authorities are facing a new challenge: fully
vaccinating those who received their first shot.
In the United States, 5
million Americans have missed their second dose of the Pfizer or Moderna
vaccine.
Or about 8% of those who
received the first dose, reveals the New York Times on Sunday, April 25.
A phenomenon that has
increased significantly in recent weeks, which worries the authorities.
Except for the Johnson
& Johnson vaccine, it takes at least two injections to be fully immunized.
"A single dose triggers a weaker immune
response and could make one more vulnerable to variants," the newspaper
recalls.
Cancelled appointments of the second dose
Such reactions were
expected, but other obstacles also disrupted the vaccination campaign.
Many Americans encountered
problems when trying to receive their second dose.:
Cancelled appointments, no
vaccine available when they arrived, Moderna instead of Pfizer or vice versa...
The Walgreens pharmacy
chain in particular would have experienced many failures in its management of
appointments and stocks.
But it assures that these
problems have been solved.
Faced with the
organizational problems, some people have probably given up.
It's not easy for everyone
to schedule their appointments on time, says Elena Cyrus, an epidemiologist at
the University of Central Florida.
"It's complicated for
people who don't have access to reliable transportation or who have jobs with
inflexible schedules. "
