The latest studies show that a person who is fully vaccinated against COVID-19 can still become infected with the virus. However, there have been many questions about whether a fully vaccinated adult can transmit the virus.
“It would be very rare. The best data we have are from long-term care facilities in a study from Denmark,” said Dr. Morgan Katz, who is an Assistant Professor of Infectious Disease at Johns Hopkins University School of Medicine in Baltimore, Maryland.
Denmark launched an immunization program against COVID-19 at the end of 2020. The Danish health authorities prioritized older adults living in long-term care facilities and frontline healthcare workers as the first receivers of vaccination. Preliminary data just published showed that 39,040 long-term care residents (average age 84 years) and 331,039 healthcare workers (average age 47 years), who were vaccinated, developed protection in very high numbers (90%) after two shots.
“This gives us a better idea because people were tested on a regular basis and so that speaks to significantly reduced risk of transmission. They looked at older adults, who are immunocompromised, so vaccines are usually less effective in these groups,” said Dr. Katz. It appears that older adults now can feel comfortable that if they are fully vaccinated they have a lower risk of contracting the virus and an even lower risk of transmitting it to others. However, Dr. Katz said that does mean these immunized adults should be in high-risk situations where there are large numbers of people who are unvaccinated and unmasked.
“In general, the vaccines are incredibly efficacious and you can feel comfortable about a very low risk of transmission, but that said, we need more long-term data,” said Dr. Katz.
Hospital epidemiologist Dr. Laraine Washer, who is an Associate Professor of Infectious Diseases at the University of Michigan Medical School in Ann Arbor, said in clinical trials the vaccines were 94% to 95% effective at preventing symptomatic COVID infection. This means that the chance of becoming symptomatic with COVID infection was 95% less, relative to people who were not vaccinated.
“The absolute risk of infection depends upon risk of exposure to COVID and that is driven by the amount of COVID circulating in the community as well as individual behaviors and interactions with others who may have COVID infection, particularly with others who have not been vaccinated,” said Dr. Washer.
The clinical trials of all COVID vaccines available in the US were nearly 100% effective in preventing need for hospitalization and death. However, fully immunized adults, who are two weeks past having received two vaccinations a month apart, in rare cases still could transmit the virus to others.
“We can’t fully answer this question yet, but the evolving data suggest that this risk is likely low,” said Dr. Washer.
Other studies suggest that the amount of virus in the nose is lower among persons who develop infection after vaccination and this might be expected to translate into a lower risk of transmission to others.
“We don’t yet have data from studies looking at household transmission where some household members are vaccinated,” said Dr. Washer.
All adults, who are fully vaccinated, should continue to wear a mask and socially distance in most circumstances, while the level of circulating virus remains high.
“Adults who are fully vaccinated may choose not to wear a mask when interacting with a small number of other asymptomatic fully vaccinated people, as long as no one else in the household is at risk for severe COVID infection and is not vaccinated,” said Dr. Washer.
Stephaun Wallace, PhD, who is a staff scientist and epidemiologist at the Fred Hutchinson Cancer Research Center and the director of external relations for both the COVID-19 Prevention Network and the HIV Vaccine Trials Network in Seattle, Washington, said if an adult has been fully vaccinated (two shots and a full two weeks after that) there is not enough data to know how common it may be to transmit the virus. However, he said that question will be answered shortly.
“Ongoing studies are assessing that now, including one that was just launched in March and involves college students on more than 20 U.S. campuses,” Wallace said.
Still testing for answers
The COVID-19 Prevention Network is conducting a study at select university campuses across the country to learn whether the Moderna vaccine stops the spread of the SARS-CoV-2 virus that causes COVID-19 disease. Over about five months, 12,000 university students will get injections of the Moderna vaccine to determine if vaccinated people can still be infected with SARS-CoV-2, and if they are able to spread the virus to other people.
The answer to this question will have a huge impact on how soon everyone can get back to school, work, and a regular social life. It is already known that this vaccine can prevent serious COVID-19 disease, but it is unknown if the vaccine will prevent the virus from shedding from the nose and mouth and infecting others. Throughout the study period, all participants will complete questionnaires via a smartphone app and provide blood samples. In addition, study participants will be asked to collect daily nasal swabs that will be batched and sent to a central laboratory for further analysis.
This will allow researchers to determine when the infection occurred, how much virus was present in the nose, and to perform genomic sequencing of the viral samples. Genomic sequencing will have the added benefit of demonstrating whether coronavirus variants of concern play a role in the vaccine’s ability to prevent infection and transmission.
The Washington State Department of Health (DOH) is investigating reports of people in the state who tested positive for COVID-19 more than two weeks after being fully vaccinated. Scientists call these “vaccine breakthrough” cases, which are expected with any vaccine. Out of one million fully vaccinated individuals in Washington State, epidemiologists report evidence of 102 breakthrough cases since February 1, 2021, which represented .01% of vaccinated people in Washington. In many ways, that is very good news and suggests that it is very unlikely for an individual who is fully immunized to transmit the virus.
