Quotes from Experts

COVID-19 risks after vaccination

SciLine reaches out to our network of scientific experts and poses commonly asked questions about newsworthy topics. Reporters can use these responses in news stories, with attribution to the expert.

Journalists: Get Email Updates

What are Quotes from Experts?

April 29, 2021


How likely is it that fully vaccinated people could be infected/reinfected with COVID-19?


“The data from both clinical trials and large ‘real world’ studies have shown that the coronavirus vaccines in use in the United States are very protective against COVID-19. Someone who has been fully vaccinated—meaning two weeks following their last shot—is unlikely to become infected if exposed to the virus and, even if they do get infected, is much less likely to experience severe symptoms, and extremely unlikely to be hospitalized. We don’t know exactly how long vaccine immunity lasts, but we do know it lasts at least six months to a year, and it could last several years.

“The vaccines are also effective against the viral variants currently prevalent in the United States. It is possible that the virus will mutate in such a way that the vaccines are less effective, in which case Americans may need to get booster shots to ensure adequate protection.” (Posted April 29, 2021)

Ashish K. Jha, PhD
Dean, Brown University School of Public Health

“No vaccine is 100% effective and vaccine breakthrough cases are expected. Recent data published from the CDC has shown 7,157 cases of breakthrough infections out of more than 87 million people fully vaccinated.  Of these, very few were hospitalized and even a smaller portion died (<1%). Recent evidence has shown that out of nearly 4,000 healthcare personnel and first responders, those who were fully vaccinated were 90% less likely to get infected. These data and others coming out show how important the vaccine is in preventing severe illness from COVID-19.” (Posted April 29, 2021)

Rachael Lee, MD, MSPH
Assistant Professor, Division of Infectious Diseases, University of Alabama at Birmingham

How likely is it that fully vaccinated people could transmit COVID-19 to unvaccinated people and what can they do to reduce the risk?


“Vaccinated people are far less likely to transmit the virus to others. Recent CDC data shows that vaccines cut transmission of the virus by around 80% as early as 2 weeks after the first dose. But nothing in life is 100%, so there is still a small risk that you can spread the virus to the people around even after you’ve been vaccinated. That’s why until we have reached herd immunity, it is important to continue the public health practices we have used during the pandemic: mask up, social distance, and wash our hands.” (Posted April 29, 2021)

Ashish K. Jha, PhD
Dean, Brown University School of Public Health

“There is a growing body of evidence that suggests that fully vaccinated people are less likely to transmit COVID-19. Recent data from Israel suggest that people who were vaccinated with the Pfizer-BioNTech vaccine who developed COVID-19 had a four-fold lower viral load than unvaccinated people. While this is only observational data, we know that viral load has been a key driver of transmission, as seen with the UK variant.” (Posted April 29, 2021)

Rachael Lee, MD, MSPH
Assistant Professor, Division of Infectious Diseases, University of Alabama at Birmingham

“It seems almost like a dream come true, but recent studies in Israel and among healthcare workers suggest that fully vaccinated people are much less likely to become infected or transmit the virus for at least several months after vaccination. It may not hold for every person who’s vaccinated, and may vary with the vaccine, but that’s still a triumph. It means that unvaccinated people can feel much safer around vaccinated people. If they get infected, it’s very likely they caught the virus from an unvaccinated person. It’s a good reason to get vaccinated as soon as possible. Having vaccines makes an enormous difference.

“We shouldn’t take this outcome for granted. Preventing infection or transmission is a high bar for a vaccine to meet, and it may be more the exception than the rule.

“We’re still in a race with the virus. Variants of the virus evolve as the virus reproduces in infected people and is transmitted. The more virus transmission, the greater risk of new variants, so we want to reduce the risk of new variants by stopping or slowing down virus transmission as much as we can. The masks and other precautions are intended to reduce transmission. If the vaccine drastically reduces the amount of virus circulating, it will slow down the appearance of new variants, along with its other benefits. We worry that eventually variants may evolve for which the vaccine no longer protects, with an ensuing ‘arms race,’ before we get enough people vaccinated. This is the familiar situation with the seasonal flu. These variants can arise anywhere in the world where the virus is actively circulating, like the South African and Brazilian variants, so that means controlling the virus everywhere in the world – vaccinating worldwide.” (Posted April 29, 2021)

Stephen S. Morse, PhD
Professor, Department of Epidemiology, Columbia University Mailman School of Public Health

“Multiple studies with a variety of vaccines—influenza, hepatitis B, typhoid and pneumonia—have shown that stress, depression, loneliness, and poor health behaviors can impair the immune system’s response to vaccines, and this effect may be greatest in vulnerable groups like the elderly. These risk factors can slow the development of a protective antibody response and may lead to earlier erosion of vaccine-produced antibodies. Obviously reducing stress and depression prior to vaccination could be helpful but may be difficult; however, the research also points to several relatively easy ways to help maximize vaccine responses in the short term. Getting a good night’s sleep prior to vaccination and exercising vigorously within 24 hours before receiving the vaccine can boost immune system responsiveness. In fact, one study showed that a 25-minute eccentric arm exercise protocol that targeted the muscle at the site of injection, performed six hours prior to an influenza virus vaccination, enhanced vaccine responses compared to those who rested quietly. Other studies show that a less sedentary and more active lifestyle, better nutrition, and low to moderate alcohol use can make a difference in immune system responsiveness to vaccination.” (Posted April 29, 2021)

Janice Kiecolt-Glaser, PhD
Director, Institute for Behavioral Medicine Research, Professor of Psychiatry and Behavioral Health, Ohio State University College of Medicine

What precautions should vaccinated travelers take to limit the risk of transmission to those still unvaccinated?


“The CDC recently released guidance for fully vaccinated travelers, suggesting that fully vaccinated travelers do not need to get tested or self-quarantine before or after domestic travel. However, to limit risk, travelers should wear masks on the flight, exercise caution around those from separate households, and wash hands. International travelers may require a negative test at their destination. While the CDC does not require it, getting tested before your travel will help to make sure that you are not carrying the disease.”  (Posted April 29, 2021)

Ashish K. Jha, PhD
Dean, Brown University School of Public Health

“I recommend reviewing the rates of COVID-19 in the area you are traveling prior to planning. Currently, the CDC has identified multiple countries at very high and high risk levels given their high rates of COVID-19. Travelers who are fully vaccinated should continue to wear masks, distance, and eat outdoors where possible to reduce risk of exposure to COVID-19 variants that may not be covered by vaccines.” (Posted April 29, 2021)

Rachael Lee, MD, MSPH
Assistant Professor, Division of Infectious Diseases, University of Alabama at Birmingham

“It’s too soon to declare victory. We all have ‘mask fatigue,’ but this is the time to be especially cautious because the virus is still out there, all over the world. Some of the best-known variants are more transmissible, meaning easier to catch, so this is a dangerous time for complacency.

“I’d recommend the now familiar precautions—mask, hand hygiene, distancing when possible—for travel, just because we don’t know what variants are out there. The vaccines work for most of them, but there are unknowns so I think it’s best to be cautious. The vaccine does give a large measure of safety, however.” (Posted April 29, 2021)

Stephen S. Morse, PhD
Professor, Department of Epidemiology, Columbia University Mailman School of Public Health

What factors should people use to determine when it is safe for them to attend large community events like concerts or sports games?


“For indoor events, I still prefer to be on the side of caution, even if vaccinated. You could consider outdoor events—with physical distancing when possible—or well-ventilated places with good hygiene and physical distancing. Risk is reduced for outdoor events, but of course is still not zero. With these precautions, we want to minimize risk as much as possible. The situation changes rapidly, so be sure to get the best information you can – CDC or your health department are good places to start. We hope to see the situation improve, at least in the U.S. and some other countries, probably in the summer, as these precautions and vaccination start to really pay off.” (Posted April 29, 2021)

Stephen S. Morse, PhD
Professor, Department of Epidemiology, Columbia University Mailman School of Public Health

“In the past year, people have been wrestling with making everyday decisions under complicated layers of uncertainty. This includes having to estimate risks even when we feel like we don’t have enough information to calculate the probability of harm. In addition, the information that we do have is constantly changing over time. This kind of ‘uncertainty about uncertainty’ is uncomfortable and very tiring. It’s tempting to think about vaccination as a way to switch off uncertainty in decision-making. Instead of flipping a switch, however, it can be useful to think about vaccination as a transition to better odds of avoiding the virus given what we know. So even if people can choose activities after vaccination that weren’t advisable before vaccination, it’s still worth recognizing if there are risks during the decision process.

“For individuals, vaccination offers reassurance about the riskiness of unfamiliar situations, and a decrease in their risk levels for many activities. Coupled with that, it’s also important for vaccinated people to keep in mind that they’re not just making choices for themselves, but for everyone else around them, including unvaccinated others.” (Posted April 29, 2021)

Uma R. Karmarkar, PhD
Assistant Professor, School for Global Policy and Strategy, Rady School of Management, University of California San Diego

Creative Commons LicenseThe text and video on this page are licensed as Creative Commons CC BY-SA 4.0. Journalists are free to use any text or video on this page with or without attribution to SciLine.

Ashish K. Jha, PhD, Dean, Brown University School of Public Health

None

Uma R. Karmarkar, PhD, Assistant Professor, School for Global Policy and Strategy, Rady School of Management, University of California San Diego

None

Janice Kiecolt-Glaser, PhD, Director, Institute for Behavioral Medicine Research, Professor of Psychiatry and Behavioral Health, Ohio State University College of Medicine

None

Rachael Lee, MD, MSPH, Assistant Professor, Division of Infectious Diseases, University of Alabama at Birmingham

None

Stephen S. Morse, PhD, Professor, Department of Epidemiology, Columbia University Mailman School of Public Health

None