Can you still spread COVID-19 after you get vaccinated?

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.

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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.

Experts note the data set was small and more results are needed. Larry Corey, a virologist at Fred Hutchinson Cancer Research Center in Seattle, called the data "suggestive" but said no conclusions should be drawn by limited testing. Dr. Corey is co-leading a federal vaccine testing program doing Phase 3 clinical trials for multiple Covid-19 vaccines and proposed a study on U.S. college campuses that was intended to examine whether Moderna's vaccine prevents the coronavirus from spreading, but the study stalled due to funding shortages and time constraints.

"As of today there really is very little information," says Dr. Corey.

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The transmission question means those getting the first vaccines aren't changing much about their behavior. Claritza L. Ríos, a 44-year-old emergency medicine physician in Oakland, Calif., received her first dose of the Pfizer vaccine on Jan. 4. Her husband, also an emergency physician, received his a week earlier. They are both due for the second dose in late January.

But her in-laws, who are in their 80's and live down the street, haven't been vaccinated yet and likely won't be for at least another month. Their two sons, ages 6 and 8, will have to wait many more months because vaccine trials in children under 12 haven't even started yet.

"Nothing's going to change until we're 100% sure that we're not going to be spreading to others," says Dr. Ríos. "We're not going to see our parents and hug them until we're 100% sure it's safe to be with them."

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.

"Most vaccines prevent disease as opposed to preventing infection," says Anna Durbin, a professor of international health at Johns Hopkins Bloomberg School of Public Health who is working on the AstraZeneca Covid-19 vaccine trial and previously worked on the Pfizer vaccine trial. She believes Covid vaccine studies will eventually show a reduction in asymptomatic transmission but not a complete elimination.

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Even if vaccines don't prevent transmission completely, they can still help populations achieve herd immunity if enough people take them, says Arnold Monto, an epidemiology professor at the University of Michigan School of Public Health who chairs the U.S. Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee. "We can still accomplish a lot even if it is demonstrated that there is still some asymptomatic infection occurring post-vaccination," says Dr. Monto.

Significant asymptomatic transmission is likely not unique to Covid-19, says Dr. Monto. Other viruses haven't been studied as extensively. When the rubella vaccine came out there were some documented asymptomatic reinfections, says Dr. Monto, but herd immunity was eventually still reached.

One potential problem is if sizable segments of the population choose not to get the Covid-19 vaccine, says John R. Mascola, director of the federal National Institute of Allergy and Infectious Diseases' Vaccine Research Center.

"Until we have broad-based vaccination and herd immunity, we should appreciate that it's possible to still get exposed to the virus really from anybody whether they're vaccinated or not," he says. But if the vast majority of people get the vaccine, "some asymptomatic transmission is not going to have much of a public health implication," he says.