Vaccinated, still got COVID? How sick will you get?
Occasional ‘breakthrough’ delta cases rarely lead to serious illness
The pandemic of the unvaccinated now threatens people who are vaccinated — but new evidence shows that these startling “breakthrough” infections tend to be milder, shorter and only occasionally spread to others.
The trend is driven by the emergence of the highly transmissible delta variant, combined with the mask-free gatherings that started after the state reopened on June 15, said Dr. Ralph Gonzales, chief innovation officer for UC San Francisco Health. Until now, vaccinated people were almost always protected from earlier variants or the original strain of the virus.
About 90% of 183 recent COVID-19 infections among UCSF staff, researchers and students are confirmed to have been caused by community exposure, the majority of which involve unvaccinated children, partners or members of the public, said Gonzales, who studies every case at the medical center.
These findings may help ease anxieties that vaccinated people may be unknow
ingly contributing to the pandemic.
Scientists are still trying to understand so-called breakthrough infections, defined as a positive test result in a vaccinated person. The problem isn’t that delta is dodging our vaccines. Instead, our bodies are just a little slow to respond to delta’s onslaught.
“The immune system takes a little while to get its boots on,” said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. Because the virus can multiply so quickly, he said, “it manages to copy itself before the immune system wakes up and stamps on it.”
The symptoms reported by people with breakthrough delta infections are different from what was seen during illnesses with the alpha variant, said Gonzales.
Among vaccinated people, the illness tends to be an upper respiratory tract infection, he said, with symptoms in the head and sinuses. People may experience sore throats, headaches and loss of smell and taste. But they rarely suffer from the lower respiratory ailments that were linked to earlier iterations of the virus, such as potentially fatal lung inflammation. At UCSF, hospitalization was only needed for one breakthrough case, and it was out of an abundance of caution. This person did not require supplemental oxygen and was discharged within three days.
“People are sick. Some of them have fevers,” Gonzales said. “But they’re all getting better.”
“Breakthroughs still feel bad,” he said. But compared to COVID-19 in the unvaccinated, “it doesn’t last as long, and it’s not as severe.”
These cases remain rare.
A new analysis of available state-level breakthrough data by the Kaiser Family Foundation found that of COVID-19 cases in California, 1.4% are in vaccinated people, and 98.6% are in unvaccinated people.
There’s a major reason why vaccinated people aren’t contributing to the outbreak: They are much less likely to be infected in the first place.
In some breakthrough cases, people may be carrying just trace amounts of virus, because their immune system has already detected and controlled it. In other cases, the virus may be abundant enough to transmit to others, but it’s controlled more quickly than in an unvaccinated person, with less time to spread.
The U.S. Centers for Disease Control and Prevention discovery of high viral loads in a recent cluster of breakthrough cases in Provincetown, Massachusetts, suggest an increased risk of transmission and raised the concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus. This finding led to the CDC’s updated mask recommendation.
But new results from the United Kingdom’s major COVID-19 monitoring program, published Wednesday, suggest that fully vaccinated people may be less likely than unvaccinated people to pass the virus on to others, due to having a smaller viral load.
“Vaccinated-to-vaccinated transmission is rare if people keep their masks on,” Gonzales said.
And a new study of breakthrough cases in Singapore also sheds light on why vaccinated and unvaccinated people don’t transmit at the same rate: Even though both groups had the same level of virus, vaccinated patients appeared to clear the virus from their bodies at a faster rate. This likely reduces the risk of transmission, because that person is contagious for a shorter period of time.
“Based on our data, it seems likely that vaccination reduces secondary transmission, though this needs to be further studied in larger community surveillance studies,” reported Po Ying Chia of the National Centre for Infectious Diseases in Singapore.
The case for vaccines remains overwhelmingly strong, say experts. Vaccines are somewhat less effective during this current surge — according to several estimates, they are about 80% effective in preventing infection by delta, down from 95% with the alpha variant. But they are almost completely effective, at 95%, in fending off serious illness.
Of 10,262 breakthrough infections in the U.S. reported in May by the CDC,
the majority were mild, according to a report released last week. About 27% were asymptomatic, 10% required hospitalization and 2% died.
The Singapore study also found that fully vaccinated patients had significantly lower odds of moderate or severe outcomes than unvaccinated patients. This finding corroborates research from the United Kingdom, which found that vaccination remains protective against symptomatic and severe disease.
Because the delta variant is now the dominant strain of COVID-19 circulating in the U.S., better surveillance of breakthrough cases is needed to monitor vaccine effectiveness — and inform discussions about booster vaccinations, said experts.
“If you’re vaccinated, you have already done the most important thing that you can do to make yourself and your community safe,” said Hanage. “But it’s not the only thing.”