The mRNA vaccines come in two doses. The first dose of the vaccine helps prepare your immune system and the second dose provides most of the immunity. Once you receive both doses of the vaccine, it will likely take several weeks for your body to develop immunity. Both doses are important to ensure full protection.
A study published in the New England Journal of Medicine in December found that protection from the Pzifer-BioNTech vaccine starts 12 days after the first shot and that it reaches 52% effectiveness a few weeks later. A week after the second vaccination, the effectiveness rate hits 95%.
Researchers are exploring whether the efficacy after the first dose is better than seen in the clinical trial. A recent study from Israel released in the Lancet, looked at 9,109 vaccinated and unvaccinated health care workers. They found:
Adjusted rate reductions for all COVID-19 infections (symptomatic and asymptomatic) were 30% for days 1-14 and 75% for days 15–28 after the first dose.
Adjusted rate reductions for symptomatic COVID-19 infections were 47% for days 1-14 and 85% for days 15–28 after the first dose.
The Moderna trial reported a protection rate of 51% two weeks after the first immunization and 94% two weeks after the second dose.
People receiving the Pfizer-BioNTech vaccine will need a second dose 21 days after the first, while those who get the Moderna vaccine will need a booster 28 days later.
Persons should not be scheduled to receive the second dose earlier than recommended (i.e., three weeks [Pfizer-BioNTech] or one month [Moderna]). However, second doses administered within a grace period of four days earlier than the recommended date for the second dose are still considered valid. Doses inadvertently administered earlier than the grace period should not be repeated.
The second dose should be administered as close to the recommended interval as possible. However, if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration up to six weeks (42 days) after the first dose. There are currently limited data on efficacy of mRNA COVID-19 vaccines administered beyond this window. If the second dose is administered beyond these intervals, there is no need to restart the series.
There has been recent discussion among public health officials, looking at population-level strategy, whether it is better to vaccinate more people with one dose (with lower efficacy) or a smaller group of people with two doses (with high efficacy). At this time, the recommendation remains that individuals receive two doses of the Pfizer-BioNTech or Moderna vaccine.
Johnson & Johnson Vaccine
The Johnson & Johnson is currently a one dose vaccine. In November 2020, Johnson & Johnson initiated a trial (ENSEMBLE 2) for a two dose regimen of the vaccine however results are not yet available.