With the United States experiencing delays in vaccination because of a shortage of staff and distribution problems, federal health officials now say it’s okay to delay the second dose of the two-part vaccine for up to six weeks.
As an infectious disease doctor, I have answered many questions from my patients, as well as friends and family, about whether the COVID-19 vaccine will still work if people delay receiving a second dose.
Why you need two doses 3-4 weeks apart
Two doses, separated by three to four weeks, is the tried and tested approach to generate an effective immune response through vaccination, not only for COVID, but also for hepatitis A and B and other diseases.
The first dose prepares the immune system and introduces the germ of interest to the body. This allows the immune system to prepare its defense. The second dose, or booster, provides the opportunity for the immune system to increase the quality and quantity of antibodies used to fight the virus.
In the case of the Pfizer and Moderna COVID-19 vaccines, the second dose increases the protection afforded by the vaccine from 60% to approximately 95%.
Why CDC decided to receive the second dose in 42 days is OK
In the clinical trial, the second dose of the Pfizer vaccine was administered on day 19 and day 42 to 93% of subjects. As the protection was approximately 95% for all who were vaccinated in this “window” of time, there are few reasons for not allowing any flexibility at the time of the second dose 2.
As more vaccine becomes available, the time for the second dose should be close to four weeks for the Pfizer and Moderna vaccines. But the good news is that even while supplies remain limited, science suggests that there is nothing wrong with receiving a second dose 42 days after the first.
What the immune system does between the first and second doses
The biology through which mRNA vaccines induce their protection against COVID-19 is fundamentally different from that with other vaccines.
The Pfizer and Moderna vaccines use messenger RNA that encodes the peak glycoprotein. After the injection of the vaccine, the mRNA enters cells of the immune system called dendritic cells. Dendritic cells use the instructions written on the mRNA to synthesize the striking peak glycoprotein, which characterizes the SARS-CoV-2 virus that causes COVID-19. These immune cells then show the peak glycoprotein to B cells, which then produce anti-peak antibodies.
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MRNA vaccines are capable of inducing a special type of immune cell – called a follicular helper T cell – to help B cells produce antibodies. T cells do this through direct contact with B cells and sending chemical signals that tell B cells to produce antibodies. It is this aid in the production of antibodies that makes these vaccines so effective.
But not all B cells are the same. There are two types of anti-thorn antibodies: long-lived plasma cells and memory B cells. Long-lived plasma cells, as the name implies, live in the bone marrow for years after vaccination, continuously producing antibodies – in this case, the anti-peak antibody. These long-lived B cells do not need reinforcement.
Memory B cells, on the other hand, live in a state similar to hibernation. They do not produce antibodies until they are stimulated by a booster vaccine, or are exposed to infection with the coronavirus that causes COVID-19. That is the reason why we need that second dose. Together, these two types of B cells provide a constant level of protection.
What happens if you don’t get your second dose of Pfizer or Moderna in time?
With the current shortage of vaccines and problems with establishing the infrastructure to vaccinate millions of people, many doctors are concerned that the second dose of the vaccine will not be delivered within the prescribed period of three to four weeks.
This booster injection is necessary for T cells to stimulate memory B cells to produce large amounts of antibodies. If booster is not delivered within the appropriate window, smaller amounts of antibodies will be produced that may not provide such powerful protection against the virus.
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