“This is the first clinical data set showing this,” Richard Webby, PhD, of St. Jude Children’s Research Hospital’s Infectious Diseases Department, tells Verywell. “We still need to treat these data as preliminary. Preliminary but encouraging!”
The Study
The analysis of the Oxford-AstraZeneca vaccine’s (AZD1222) ongoing trial was published as a preprint (meaning it is still under review) in The Lancet. One of the study’s most intriguing findings was that the vaccine might have a “substantial effect on transmission.”
According to Oxford, the robust efficacy after the initial shot supports the strategies that some nations are using, like spacing out the second dose to 12 weeks, to get as many first doses of the AstraZeneca vaccine to citizens as quickly as possible.
Participants in the United Kingdom arm of the study were asked to produce nasal swabs each week. Next, the samples underwent polymerase chain reaction (PCR) testing to detect the presence of SARS-CoV-2 (the virus that causes COVID-19).
“When you have an effective vaccine and it mounts a good immune response [and] a person is exposed to the virus, the strength of the immune response will dictate course of the disease and how fast the virus is cleared from the system.” Jagdish Khubchandani, PhD, professor of public health at New Mexico State University, tells Verywell. “This, in turn, would relate to transmissibility.”
However, after two doses of the vaccine, researchers found only a 49.5% reduction in positive tests.
“The paper is still under review, so there is that,” says Khubchandani, emphasizing the need for more research.
Other COVID-19 Vaccines and Transmission
In the United States, there are currently only two COVID-19 vaccines that have been granted emergency use authorization (EUA) by the Food and Drug Administration (FDA): the Pfizer-BioNTech and Moderna vaccines.
However, in a press release, Johnson & Johnson recently provided preliminary data from its Phase 3 clinical trial for the Janssen COVID-19 vaccine. On February 4, the company said that it had asked the FDA for an EUA.
The vaccine manufacturers have not made explicit claims about the shots’ ability to slow or cut transmission, but that information may come later as more research is undertaken.
Khubchandani says that “theoretically, any highly-effective vaccine should reduce symptomatic/asymptomatic carrying of virus and its transmission. But proof is still needed in practice.”
That said, there are limitations to what can be expected of any vaccine. “They will almost certainly not reduce transmission to zero,” Webby says. “But these vaccines should both protect the vaccinated person from being infected, and where they don’t do that, reduce the amount of virus they [shed]. Both will reduce transmission.”
“There have also been some limited data for Moderna’s vaccine that were included in its FDA briefing document,” Michael Haydock, therapeutic area director at Informa Pharma Intelligence, tells Verywell. He adds that the data from Moderna “suggested a ~63% drop in asymptomatic infections after the first dose in vaccinated subjects vs control subjects.”
Why We Have to Slow the Spread
Efficacy rates are important to spare people from severe illness and death from COVID-19 and to ease the burden on the healthcare system. However, stopping virus spread is also a huge factor in getting to the other side of the pandemic.
According to Johnson & Johnson, its single-dose vaccine shows 85% efficacy in preventing severe disease across all regions studied.
“It is key that we vaccinate people faster,” Khubchandani says. “And also have vaccines that reduce transmission from person to person—even from the vaccinated people.”
If someone who is vaccinated cannot transmit the virus, that adds another element to vaccine protection. Not only will vaccination protect the individual receiving the shot, but it also helps to protect anyone that person comes into contact with who have not been vaccinated.
“There are always going to be individuals that remain susceptible to the virus even with the vaccine,” Webby says. “They will not be 100% effective. By reducing transmission, we also protect this part of the population. If we hope to eradicate the virus, reducing transmission is key.”
Does it Matter Which Vaccine You Get?
“We are running at a slow pace compared to the virus,” Khubchandani says. “COVID-19 virus spread has been like a wildfire. So we have to be fast, resourceful, be able to use multiple techniques.” Those multiple techniques, he says, include using a menu of vaccines.
As the list of available vaccines expands, some people may wonder if they should wait to see if they’re able to get the vaccine than the one first available to them.
“At this stage, it’s not appropriate to compare one against the other,” Webby says. “All we can say is that they all do really, really well, and people should absolutely take whatever they have access to first.”
Haydock recommends that people “receive whatever vaccines are available at the time.” He adds that all the vaccines that are currently available have been shown to have “good safety profiles and have strong evidence that they are effective at preventing severe infections and hospitalizations.”
The National Institutes of Health (NIH) estimates that 75% to 80% of Americans must get a COVID-19 vaccine before herd immunity is achieved and a “semblance of normal life” can be resumed.
“People need to choose vaccine over disease,” Khubchandani says. “Even a single dose of any vaccine is better than one-time infection with COVID-19.”
So far, according to the Centers for Disease Control and Prevention’s (CDC) COVID Data Tracker, more than 32 million Americans have received their first vaccine dose—only about 10% of the population.
Khubchandani says that it’s also important to continue to dispel fears about getting vaccinated. “I have worked in medical schools, basic science labs, and now in the field of behavioral and social sciences. No one wants to harm people,” he says. “No person in a scientific profession thinks like that. There is rigorous training and oversight in whatever is being done with vaccine development.”
The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.
