Recent studies suggest that the Moderna and Pfizer vaccines are up to 90 percent effective against COVID-19 infections after full immunization. Researchers in the field note that even after just a single dose, these vaccines can be up to 80 percent effective in preventing COVID-19 infection. Despite the apparent effectiveness after just one shot, experts still urge vaccinated individuals to receive the second dose — if required — on time and follow public health measures, such as masking and social distancing, to slow community transmission.
As of Tuesday, approximately 142 million Americans have received at least one dose of the vaccine, and 96 million are fully immunized. According to the Virginia Department of Health, over 3.7 million Virginians have received one dose, and 2.5 million are fully vaccinated. As vaccination rates in the general public increase, it is important that individuals understand their level of protection from the coronavirus and recognize that they should still adhere to public health measures outlined by the CDC, even if they are fully vaccinated.
Unlike the Johnson & Johnson vaccine, the Moderna and Pfizer vaccines require two doses due to a difference in the biological foundations of the vaccine as well as differences in study design. Researchers developing the Johnson & Johnson vaccine studied the vaccine in a single-dose format and were satisfied by the effectiveness of just one shot. The Johnson & Johnson are also vaccines that use adenoviruses as a mechanism to instruct cells to make the adenovirus.
According to Dr. Steven Zeichner, professor of pediatrics, microbiology, and immunology at the University, this is another reason the Johnson & Johnson vaccine should be administered in just one dose.
“When you give somebody an adenovirus vector vaccine, they will make an immune response not only against the SARS-CoV-2 instructions that are inserted into the cell, but they will also … make an immune response against the adenovirus vector itself,” Zeichner said. “So if you have a preexisting immune response against an adenovirus vector, the next time you see that your body may fight that adenovirus vector and make it harder for it to elicit an immune response.”
In contrast, both the Moderna and Pfizer are mRNA-based vaccines that use the genetic material mRNA to encode a spike protein in immune cells. This spike protein is also found on the surface of the coronavirus. Thus, by introducing the spike protein to our own cells, the immune system can begin to form an immune response to the coronavirus and produce antibodies, protecting vaccinated individuals from future COVID-19 infection.
The Moderna and Pfizer study designs also revolved around a two-dose regimen, which is why they must be administered in a two-dose format. Dr. William Petri, professor of infectious diseases at the University, explained the functions of each of the two doses of the mRNA vaccines.
“What's the first shot is doing is it's introducing the spike glycoprotein … of the virus to the immune system,” Petri said. “When it sees the same spike glycoprotein approaching again with a second shot, then it reactivates and makes more antibodies, probably higher affinity antibodies that bind even tighter to the spike glycoprotein, and so the second shot is going to increase the protection of the vaccine.”
By standards established by the Centers for Disease Control and Prevention in collaboration with vaccine developers, individuals vaccinated with the Pfizer vaccine should wait 21 days between the first and second shot while the recommended interval between doses of the Moderna vaccine is 28 days. However, both Petri and Zeichner acknowledge that this is not a strict schedule, and missing the deadlines should not significantly lower the effectiveness of the vaccine. CDC guidelines state that “up to 42 days between doses is permissible.”
“If there's a little bit of slippage or something like that, I don't think there's going to be irreparable harm done to the patients and a lot of this is … a little bit arbitrary,” Zeichner said. “Ideally, everybody should be vaccinated according to the approved schedule … but it's not horrible if you don't. It doesn't mean that you're not going to get protection.”
Another important reason individuals should make it a priority to get the second dose of the mRNA vaccines and within a reasonable schedule is to halt the spread of existing COVID-19 variants and prevent the emergence of new variants. According to Zeichner, only giving a single dose of an mRNA vaccine to the majority of the population before giving the second dose is a recipe for the development of new and more persistent variants.
“If you have a lot of circulating virus out in the population, where you may have people who have some immunity, but not sufficient immunity to prevent the people from getting that infection, that's a setup for the evolution of a virus that may be resistant to the immune responses that the vaccines are designed to elicit,” Zeichner said.
So far, all three vaccines approved for use in the U.S. appear to be effective against the U.K variant, though more studies are needed to concretely prove the efficacy of the vaccines against South Africa’s variant and the Brazilian variant.
Both Zeichner and Petri point out the key to preventing a situation in which a new variant is resistant to the effects of the vaccine is by decreasing community transmission. Even when an individual is vaccinated, behaviors in public and attitudes toward public health measures should not change, except in small gatherings with other vaccinated individuals.
“Until pandemic transmission stops, then we should all be wearing masks, we should all be socially distancing, we shouldn't be eating indoors, all of those things,” Petri said. “Sometime in the future or something where there's not 60,000 cases a day, then people who are vaccinated can kind of let down their guard more.”
Some vaccinated students at the University seem to agree. First-year Engineering student Naomi Nichols is fully vaccinated against COVID-19 but says she will likely not change her behaviors until the pandemic is under control.
“What I was told and that kind of impression that I got was that getting the vaccine kind of protects you from getting a serious case of COVID,” Nichols said. “But I could still have it and transmit it, so I definitely don't want to get it and then not know and then give it to people. So I probably won't change a whole lot. But there's a good kind of peace of mind that if I get it, I'm okay.”
According to CDC recommendations, vaccinated individuals can gather with other vaccinated individuals in small groups without masks, but it is still paramount that COVID-19 measures are still followed in public spaces. Active cases within the U.S. still remain quite high, with the current daily average standing at 64, 152. Although the vaccines, even from just a single dose, are highly effective, in order to fully resume our lives as they were before the pandemic, we must continue to obey public health measures until community transmission decreases and the country reaches herd immunity.