Do the Covid vaccines prevent you from spreading the virus, or do they just protect you from getting sick?

“Scientists don't know yet -- and the uncertainty has big implications during the rollout of the vaccines.

Pfizer and Moderna, the companies that developed the vaccines authorized in the U.S. so far, say their vaccines are about 95% effective at preventing people from getting sick with Covid symptoms. But there's not enough evidence yet on whether the vaccines also prevent asymptomatic infection and transmission.

The companies say research is ongoing to determine the answer. Without vaccines, research has suggested that asymptomatic transmission of the coronavirus that causes Covid is responsible for roughly a quarter of infections.

The result, experts say, is that precautions like wearing masks, social distancing and avoiding crowded spaces will be necessary until the country gets closer to herd immunity, the point at which enough people are immune to a disease to make its spread unlikely. Some studies have estimated roughly 75% to 80% of the U.S. population needs to be immune to Covid-19 to reach herd immunity, but that number is a moving target and could rise as new variants emerge.

"Everyone needs to keep wearing masks and we all need to do our part in reducing the transmission so it's not going to be as difficult to control," says Marion Pepper, an immunologist and associate professor at the University of Washington in Seattle. "This is really important."

There is some indication that vaccination may reduce asymptomatic infection, resulting in reduced transmission. Preliminary evidence from Moderna showed that participants in a clinical trial who received the vaccine and were tested for Covid between their first and second doses had a roughly two-thirds reduction in asymptomatic infections.

"That means there's a substantial, substantial reduction in overall infections just from that first dose," says Deepta Bhattacharya, an associate professor of immunobiology at the University of Arizona in Tucson.

The coronavirus enters the body typically through the nose or mouth, says Dr. Bhattacharya. But the most severe disease from Covid-19 usually occurs in the lungs. Vaccines are injected into the bloodstream, and antibodies develop in the blood before moving to the nose to prevent infection. "Antibodies can cross into the lungs a little bit more easily than they can in the nose or throat," says Dr. Bhattacharya. "So it's a lot easier to prevent severe or symptomatic disease" than infection, he explains.

Even after getting vaccinated, if someone is exposed to the virus it can take the body's immune response some time to control an infection, says Dr. Pepper. The potential for transmission depends on how quickly the infection is controlled.” [1]

 

1. Health & Wellness -- Your Health: Can You Still Spread the Virus After Vaccination?

Reddy, Sumathi. Wall Street Journal, Eastern edition; New York, N.Y. [New York, N.Y]12 Jan 2021: A.12.