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Covid vaccine: what is the purpose of the second dose?

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.


Covid vaccine: what is the purpose of the second dose?


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. "


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