- COVID-19 vaccines continue to protect against severe illness but do not entirely block transmission.
- Fully vaccinated people are also less likely to contract the coronavirus than unvaccinated people.
- Experts emphasize that wider vaccine coverage is needed to ensure that when people have an infection, they are well protected against severe COVID-19.
COVID-19 vaccines continue to work, even in the face of the highly infectious Delta variant — especially when it comes to protecting against severe illness and death.
Fully vaccinated people are also less likely to contract the coronavirus than unvaccinated people. If they don’t contract an infection, they can’t transmit the virus to others.
However, a recent studyTrusted Source shows that when fully vaccinated people do contract the coronavirus, they can still transmit it.
Experts say these new results, which were published Oct. 29 in The Lancet Infectious Diseases, should not deter people from getting vaccinated.
Instead, they emphasize that wider vaccine coverage is needed to ensure that when people have an infection, they are well protected against severe COVID-19.
In the new study, researchers focused on the transmission of the Delta variant within households, a common setting for coronavirus transmission.
Researchers analyzed data from 204 household contacts of 138 people with a Delta infection.
They found that household contacts who had received two doses of a COVID-19 vaccine were less likely to contract an infection with the Delta variant than unvaccinated people.
According to the analysis, 25 percent of vaccinated contacts exposed to a household member with an infection contracted one themselves.
In contrast, 38 percent of unvaccinated contacts got an infection.
This is in a household setting, where people are in close contact for extended periods.
“These results underpin the key message that vaccinated contacts are better protected than the unvaccinated,” Dr. Annelies Wilder-Smith, a member of The Lancet Commission on COVID-19 and a consultant to the World Health Organization, wrote in an accompanying editorialTrusted Source.
The study was carried out in the United Kingdom, so all study participants were vaccinated with either the Pfizer-BioNTech or AstraZeneca COVID-19 vaccine.
Vaccinated people can contract and transmit Delta
Although fully vaccinated people were less likely to contract an infection, when they did — what’s known as a breakthrough infection — they can transmit the Delta variant at a similar level as unvaccinated people.
Researchers found that 25 percent of household contacts exposed to a fully vaccinated person in the household contracted an infection themselves.
Of those exposed to an unvaccinated household member, 23 percent contracted an infection.
“Breakthrough infections in fully vaccinated people can efficiently transmit infection in the household setting,” wrote the study authors.
Researchers suspect this has to do with the coronavirus replicating similarly in vaccinated and unvaccinated people — at least at the start of the infection.
As part of the study, researchers also measured the viral load — how much virus is in the body — of people who contracted an infection.
The peak viral load was similar for both vaccinated and unvaccinated people. It was also similar for people with an infection with different variants.
However, there was a slight increase in viral load with increasing age. This suggests a weaker immune response in older people.
Vaccination helps people clear virus more quickly
Even though the peak viral load was similar for vaccinated and unvaccinated people, the viral load decreased faster for fully vaccinated people with a Delta infection than for unvaccinated people.
“This study confirms that COVID-19 vaccination reduces the risk of delta variant infection and also accelerates viral clearance in the context of the delta variant,” Wilders-Smith wrote.
Viral load is directly related to infectiousness. Higher viral loads are more likely to lead to transmission of the virus.
The study results suggest that because the viral load of vaccinated people drops off more quickly, their infection may be infectious for a shorter time than for unvaccinated people.
The researchers, though, didn’t look specifically at how likely people were to transmit the virus during the later stages of their infection.
COVID-19 vaccines protect against severe illness
The new study looked only at people with mild COVID-19, along with their risk of transmitting the coronavirus to other household members.
But other research shows that while COVID-19 vaccines may be less protective against infection caused by the Delta variant — compared with earlier variants — they still protect against severe disease.
“What we’re seeing in the hospital — especially in our critically ill patients, and even in those that are not critically ill, but are hospitalized — is that they’re primarily unvaccinated patients,” said Dr. Mohammad Sobhanie, an infectious disease physician at The Ohio State University Wexner Medical Center.
COVID-19 vaccines could have kept many of these people out of the hospital.
A report by the Kaiser Family Foundation estimates that more than 280,000 COVID-19 hospitalizations in the United States between June and August 2021 could have been prevented by vaccination.
Regularly updated data from the Centers for Disease Control and Prevention (CDC) also shows that unvaccinated people are 11.3 times more likely to die from COVID-19 than fully vaccinated people.
Vaccinating people is high priority
Most experts are not surprised that the current COVID-19 vaccines do not prevent transmission of the coronavirus.
That was never the main purpose of these vaccines.
Instead, they are intended to prevent severe illness and death — which they continue to do well.
But several groups of researchers are working on vaccines that they hope will block infection from the start — what’s called sterilizing immunity.
These vaccines are delivered intranasally (in the nose). Animal and early clinical trials show that this type of vaccine can provide local immunity against the coronavirus.
Intranasal COVID-19 vaccines still have to be tested in larger clinical trials, so it will be a while before we can block all coronavirus transmission.
In the meantime, Sobhanie said the best way for people to protect themselves and others is to get vaccinated with the COVID-19 vaccines that are already available.
This will make for a safer holiday season, when many people will be gathering indoors with family and friends for meals and festivities.
With the Pfizer-BioNTech pediatric vaccine now available in the United States for kids ages 5 to 11, more people at family gatherings can be protected.
But infants and toddlers are still not eligible. Results from vaccine clinical trials for this age group may not be available until early next year.
In addition, some people who are fully vaccinated may still be at a higher risk of COVID-19 due to their age or underlying medical condition.
“While gathering for the holidays, we do have to be cognizant of relatives who may be elderly,” Sobhanie said. “We also need to be cognizant of relatives or family members who are immunocompromised, [such as] those who are undergoing active chemotherapy.”