As the highly transmissible Delta variant spreads, more vaccinated Americans are reporting they have tested positive for Covid-19. Here’s what you need to know about so-called ‘breakthrough’ cases:
What is a breakthrough infection?
A breakthrough infection is when someone who received a Covid-19 vaccine contracts the virus more than 14 days after they have been fully vaccinated.
Breakthrough infections are rare and generally mild. Most don’t require hospitalization, according to Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University Medical Center.
Who is most at risk for breakthrough infections and cases?
Doctors say the elderly and those with compromised immune systems are most at risk for breakthrough infections.
This is because aging impairs the body’s intrinsic ability to adapt to an immune response, according to Dr. Akiko Iwasaki, an immunologist at Yale University.
People who are immunocompromised due to an underlying illness are also at risk because they may not be able to generate the amount of antibodies required to fight infection altogether, she said.
How many people are getting breakthrough infections?
Doctors and immunologists say these infections are relatively rare.
A report from late May by the CDC found more than 10,000 cases of Covid-19 in vaccinated people at least 14 days after their final shot across 46 states and territories between Jan. 1 and April 30 of this year. That equates to about 0.01% of the roughly 100 million people who were fully vaccinated as of April 30.
Of those roughly 10,000 people, about 10% were hospitalized. Of that 10%, nearly a third didn’t show symptoms or were in the hospital for a reason unrelated to the coronavirus; 2%—or 160 patients, died.
The median age of patients who died after being diagnosed with Covid-19 post-vaccination was 82 years old, the CDC found. And 18% of those who died passed away due to reasons that weren’t related to Covid-19.
Since that report, the CDC stopped tracking breakthrough cases of Covid-19 that didn’t lead to hospitalization or death. As of July 19, the CDC recorded 5,914 cases across 49 states and territories of patients with the Covid-19 vaccine that were hospitalized or died. Of those cases, 74% were in people over the age of 65. Among the full group there were 1,141 deaths. The CDC said 26% of those fatalities were among people who were asymptomatic or who died from something not related to Covid-19.
What are the symptoms of a breakthrough Covid-19 infection?
Vaccinated people who have breakthrough Covid-19 infections may not know that they are infected, since some show no symptoms. Olympic athletes and others who are regularly tested have been surprised by positive Covid-19 results, since they didn’t feel sick.
For others who do feel sick, the symptoms are often shorter in duration and milder than those who contract Covid-19 without a vaccine. Those symptoms include cough, fever, headaches and fatigue, according to a preprint study published in late May in the U.K. by researchers at King’s College London.
How protective are the vaccines against the Delta variant?
Covid-19 vaccines are holding up well against the highly contagious variant, although they were built for the original strain.
The vaccines that were authorized for use in the U.S. were tested for their ability to prevent symptomatic disease, rather than completely prevent infection. They have done well so far in protecting people from severe illness or hospitalization, doctors say.
A report in July published in the New England Journal of Medicine found that vaccines from Pfizer Inc. and BioNTech SE as well as AstraZeneca PLC were highly effective against the Delta variant after two doses.
The study, which involved almost 20,000 people, found Pfizer was 88% effective at preventing symptomatic disease from Delta, compared with 36% after one dose. The effectiveness of AstraZeneca’s shot was 67% after both doses, compared to 30% with one dose, according to the study, which was sponsored by Public Health England.
A smaller, preliminary study released in July by the government of Israel found that after two shots the vaccine was 39% effective at reducing the risk of infection and 40% effective at reducing the risk of symptomatic disease during a period when the Delta variant dominated cases in Israel. It was still 91% effective at preventing severe illness.
Moderna said it found promising evidence that its shot performed well against emerging variants, including the Delta strain. The company analyzed blood samples by vaccinated individuals.
Johnson & Johnson said that a small lab study showed signs that its vaccine triggered a strong immune response against the Delta variant, as part of the analysis of blood samples from eight vaccinated individuals.
Can vaccinated people be infectious to others?
Generally, people who are vaccinated are much less likely to transmit the virus to others. The highly contagious Delta variant has complicated matters.
A June report in the New England Journal of Medicine found that the viral load among vaccinated people was some 40% lower than those who didn’t get inoculated.
However, variants like the Delta strain—which can have 1,000 times more particles of the virus than the original Covid-19 strain—can make it more likely for those with the shot to both contract and spread the virus, compared with earlier versions of Covid-19.
This story has been published from a wire agency feed without modifications to the text
