In a stunning demonstration of human endeavor, less than one year after a new virus swept across the world triggering a pandemic, we developed several effective vaccines that protect those inoculated from hospitalization and death. However, one key question not answered by rigorous clinical trials last year was whether the vaccines prevent asymptomatic infection, and onward transmission.
While it was clear the vaccines could dramatically reduce rates of death from COVID-19, it was unclear whether they could prevent mild, or even asymptomatic infections. Now, as millions have been vaccinated around the world, researchers are getting some promising early indications that mRNA vaccines in particular may successfully prevent SARS-CoV-2 infection and subsequent transmission.
Over 100 million Americans have received at least one dose of a mRNA COVID-19 vaccine (either from Pfizer or Moderna). A new Mayo Clinic-led study, published in the journal Clinical Infectious Diseases, looked at retrospective data from nearly 40,000 subjects who took a routine pre-operative COVID-19 test over the past couple of months. About 3,000 of those subjects had received at least one dose of an mRNA vaccine more than 10 days before the COVID-19 test.
“Among individuals who had received a single dose of vaccine [more than] 10 days prior to their pre-procedure test, we observed a 72 percent reduction in the risk of a positive molecular screening test,” the researchers report in the study. “After adjustment for multiple potential confounding factors, we observed an 80-percent reduction in the risk of a positive molecular screening test among test performed in persons who had received two doses of vaccine, compared to those who were not vaccinated.”
These findings promisingly echo several other studies appearing on pre-print journal servers in recent weeks. A UK investigation tracking healthcare workers in the weeks following a single dose of the Pfizer mRNA vaccine found a 75-percent reduction in asymptomatic cases compared to a matched unvaccinated cohort.
Researchers have been closely studying case numbers in Israel, as that country is leading the world in mass vaccinations. Early indications are again suggesting asymptomatic infections may drop by as much as 75 percent two weeks after one dose of the Pfizer vaccine.
In mid-February Anthony Fauci noted during a White House COVID-19 briefing that preliminary data looking at the first wave of vaccinations is beginning to suggest mRNA vaccines don’t just prevent severe disease, but also prevent infection.
"The looming question is, if the person who's been vaccinated gets infected, does that person have the capability to transmit it to another person," said Fauci. "Some studies are pointing in a very favorable direction."
Another major clue these early mRNA vaccines can prevent onward transmission of the virus are preliminary studies tracking reductions to overall viral loads in vaccinated subjects who do eventually become infected with SARS-CoV-2. One study, still in preprint and not yet peer-reviewed, found viral load to be “four-fold” lower in subjects infected 12 to 28 days after one dose of the Pfizer vaccine.
A Lancet study looking at transmission of the virus in Spain confidently noted a correlation between viral load and increased transmission. Fauci suggests this indicates a person is much less infectious when they present with a lower viral load.
"There's a direct correlation with viral load and transmission," Fauci said last month. "In other words, higher viral load, higher transmissibility; lower viral load, very low transmissibility."
All of these findings build a confident and promising picture of mRNA COVID-19 vaccines effectively reducing viral transmission. Of course, the picture won’t become clearer until more people are vaccinated, and there are still unanswered questions over how effective these vaccines are against emerging SARS-CoV-2 variants. But overall, the real-world data from these first months of mRNA vaccinations are extremely promising and serve as a reminder for all to get vaccinated, regardless of personal risk, as soon as the opportunity arises.
The new study was published in the journal Clinical Infectious Diseases.
Source: Mayo Clinic
