COVID-19 vaccine FAQ: Can vaccinated people still spread the coronavirus? Should they wear masks?

LOS ANGELES - It may sound simple: Get the COVID-19 vaccine and now you’re protected from getting sick. But can you still get infected and transmit the coronavirus, which has claimed more than 300,000 American lives, even after being vaccinated?

Last week, the first vaccine by Pfizer and BioNTech was given emergency use authorization by the U.S. Food and Drug Administration and shipments to the nation’s hospitals quickly began. The first 3 million shots were strictly rationed to front-line health workers and elder-care patients.

A key advisory committee to the FDA Thursday voted to recommend the approval of Moderna’s COVID-19 vaccine for emergency use, which means it could become the second shot to be approved by the agency and to be distributed across the United States.

Both Pfizer and Moderna’s vaccines have shown promising results in their phase three trials with 90 to 95 percent efficacy.

"What this efficacy means is protection against COVID disease — that could be mild, moderate and severe disease. The trials were primarily designed to see whether these vaccines protect against diseases," Dr. William Moss, a faculty expert at John Hopkins Bloomberg School of Public Health and an executive director at the International Vaccine Access Center, told FOX TV Stations. "The fact that we have a vaccine a year into this and that we have such protective vaccines is just astonishing."

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The vaccine trial results came from 30,000 to 45,000 volunteers from each study. Researchers tested whether the participants showed signs or symptoms of COVID-19 post-vaccination and then tested the individuals with a nasal swab to see whether they could confirm that the individual was infected with the coronavirus.

While the trials tracked how many people became sick with COVID-19 after taking the vaccine, questions about whether the vaccines offer protection from infection by the coronavirus remain.

Can people who are vaccinated be infected by the coronavirus?

"There remain to be a number of unanswered questions," Moss shared.

For example, could individuals who are vaccinated still become silently infected, transmitting the virus to others without knowing it?

"The short answer is that we don’t yet know," Moss said. "What we can assume is if these vaccines are protecting against disease, that individuals, if they were infected, are going to have a lower level of virus."

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Moderna found a hint that its vaccine may prevent symptomless infection. Study participants had their noses swabbed prior to the second dose of either vaccine or placebo. At that one timepoint, swabs from 14 vaccine recipients and 38 placebo recipients showed evidence of asymptomatic infection, said Moderna’s Dr. Jacqueline Miller.

Meanwhile, a study released in November suggested that some people may generate a level of COVID-19 "immune memory" that could offer some protection from the coronavirus after infection, even after six to eight months.

"Most people are making immune memory for at least six-plus months and it looks like a type of immune memory that probably is capable of keeping most people from getting serious disease again," Dr. Shane Crotty, an immune and vaccine scientist and senior co-author of the study, said.

Another vaccine trial by AstraZeneca, which announced some of its trial results in November, used different approaches to inoculating the people who participated, and its results suggest that a vaccine could prevent some infection.

"They were able to see in some early analyses of their phase three trial that it looks like the vaccine did prevent asymptomatic infection — that infection without symptoms — and that very likely is going to mean that those individuals cannot transmit the virus. So they do have some evidence of that," Moss said.

AstraZeneca’s vaccine candidate efficacy averaged about 70 percent, according to the company.

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But both AstraZeneca and Moderna’s results still do not — with certainty — indicate whether a vaccinated individual could be spreading the virus to others.

"We don’t yet know if these vaccines actually prevent what we call asymptomatic infection, infection without symptoms, and whether they actually prevent someone from transmitting the virus to another person," Moss said.

Moss said it’s a point that will need to be further studied going forward, and the hope is that that more will be known in the months to come as vaccine trials continue to progress.

Should people still wear masks after they’re vaccinated?

If there’s a chance that vaccinated people could still spread the coronavirus asymptomatically, the need for masks will not subside as the vaccines are rolled out.

Essentially, vaccinated people could potentially be putting unvaccinated people at risk, Moss acknowledged. 

"People are going to continue to need to wear masks, physically distance, wash our hands frequently, avoid large gatherings, certainly in the coming months until a large proportion of the United States population is vaccinated," Moss said. "We’re going to need to continue those basic public health measures, even as these vaccines are rolled out."

Dr. Jason McKnight, a primary care physician at Texas A&M University, echoed Moss’ advice, pointing out that wearing a mask is not only a barrier against COVID-19, but other viruses as well.

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"Continuing to wear a mask may help prevent the spread of other respiratory illnesses, which can help prevent overwhelming the health care system, as we are already seeing during the pandemic," McKnight said.

The U.S. Centers for Disease Control and Prevention still maintains that wearing a mask, socially distancing at a minimum of 6 feet apart and washing your hands are important steps to slowing the spread of the virus.

While 95 percent vaccine efficacy sounds reassuring, it still means that the vaccine did not work for 5 percent of individuals, suggesting that following the CDC guidelines is even more imperative.

"The vaccine’s not 100 percent effective," Moss said. "So, until we get this pandemic under control, until the transmission is reduced, there’s still some people who have been vaccinated who may still get COVID-19."

What side effects can be expected from the vaccines?

Moss noted that there are some normal side effects that are expected following vaccination.

"It’s our body’s immune system responding to the vaccine. These are common and we’ve seen these with both the Pfizer and the Moderna vaccine — soreness at the sight of injection, some muscle aches, headaches, joint pains, some people will have fever. These are all expected side effects of the vaccine."

According to History of Vaccines, an educational resource provided by the College of Physicians of Philadelphia, most side effects from vaccinations are mild.

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Common side effects include soreness or swelling at the injection site, fever, rash and achiness. But in rare cases, side effects can be severe, such as a life-threatening allergic reaction.

The side effects reported in the Pfizer and Moderna vaccine trials are in line with the common adverse events.

However, some individuals have suffered allergic reactions after receiving Pfizer’s COVID-19 vaccine. Last week, British regulators said two health care workers who received the Pfizer coronavirus vaccine had severe allergic reactions.

RELATED: Alaska health care worker suffers adverse reaction after COVID-19 vaccine

Meanwhile, a health care worker in Alaska, who was identified as a middle-aged woman, had an allergic reaction that included flushing and shortness of breath within 10 minutes of receiving the first of Pfizer's two-dose jab on Tuesday.

"We still don’t know what component of the Pfizer vaccine is really inducing this anaphylactic reaction — this severe allergic reaction that we’ve seen — right now there’s no plans to delay or change how we vaccinate," Moss said. "We’ll learn more going forward about what in the vaccine is inducing these allergic reactions and whether we’ll see them with the Moderna vaccine as well."

Will vaccines end the pandemic?

Going forward, Moss says that researchers want to continue to learn more about the potential impacts of the vaccines.

"We need to better understand how long the protection lasts, the duration of protection, and that we’re only going to learn as we continue to follow the participants in the trial and observe people who are vaccinated," Moss said. "We’re going to need to learn how these vaccines work and whether they’re safe in pregnant women and in younger children so those individuals can get vaccinated."

But Moss says there is hope.

"We have hope that we’re going to make it through this pandemic. It’s going to take time. There’s going to be bumps until we get everyone who needs or wants to be vaccinated in the United States, but we see a light at the end of the tunnel. But that means we really need to be very careful and protect ourselves and people around us through the other basic public health measures so we can get to that point in the vaccine where we can start getting our lives back to normal," Moss said.

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Moss noted that researchers will need to learn more about how to vaccinate large numbers of people in the United States, as nothing has ever been done at such a large scale.

Moss said the messaging around the vaccines is also critical to widespread adoption.

"There’s going to be rumors, there’s going to be misinformation and disinformation, and we need to be prepared to get the right messages out — the correct information out about these vaccines," Moss said.

The Associated Press contributed to this story..