The Mercury News

Vaccinated, still got COVID? How sick will you get?

Occasional ‘breakthrou­gh’ delta cases rarely lead to serious illness

- By Lisa M. Krieger lkrieger@bayareanew­

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.”

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