People with breakthrough COVID-19 infections are two times more likely to be completely asymptomatic and are about two-thirds less likely to be hospitalized, compared with those who are unvaccinated, according to a new observational study.

Individuals infected with COVID-19 after receiving their first or second dose of either the Pfizer, Moderna, or AstraZeneca vaccine experienced a lower number of symptoms in the first week of infection, compared with those who did not receive a COVID-19 vaccine, reported the authors of the report in The Lancet Infectious Diseases. These patients also had a reduced need for hospitalization, compared with their unvaccinated peers. Those who received both doses of a vaccine were less likely to experience prolonged COVID - defined as at least 28 days of symptoms in this paper - compared with unvaccinated individuals.

“We are at a critical point in the pandemic as we see cases rising worldwide due to the delta variant,” study co–lead author Dr. Claire Steves, said in a statement. “Breakthrough infections are expected and don’t diminish the fact that these vaccines are doing exactly what they were designed to do – save lives and prevent serious illness.”

For the community-based, case-control study, Dr. Steves, who is a clinical senior lecturer at King’s College London, and her colleagues analyzed and presented self-reported data on demographics, geographical location, health risk factors, COVID-19 test results, symptoms, and vaccinations from more than 1.2 million UK-based adults through the COVID Symptom Study mobile phone app.

They found that, of the 1.2 million adults who received at least one dose of either the Pfizer, Moderna, or AstraZeneca vaccine, fewer than 0.5% tested positive for COVID-19 14 days after their first dose. Of those who received a second dose of a COVID-19 vaccine, 0.2% acquired the infection more than 7 days post vaccination.

Likelihood of severe symptoms dropped after one dose

After just one COVID-19 vaccine dose, the likelihood of experiencing severe symptoms from a COVID-19 infection dropped by a quarter. The odds of their infection being asymptomatic increased by 94% after the second dose. Researchers also found that vaccinated participants in the study were more likely to be completely asymptomatic, especially if they were 60 years or older.

Furthermore, the odds of those with breakthrough infections experiencing severe disease – which is characterized by having five or more symptoms within the first week of becoming ill – dropped by approximately one-third.

When evaluating risk factors, the researchers found that those most vulnerable to a breakthrough infection after receiving a first dose of Pfizer, Moderna, or Astrazeneca COVID-19 vaccine were older adults (ages 60 years or older) who are either frail or live with underlying conditions such as asthma, lung disease, and obesity.

The findings provide substantial evidence that there are benefits after just one dose of the vaccine, said Diego Hijano, MD, MSc, pediatric infectious disease specialist at St. Jude’s Children’s Research Hospital, Memphis. However, the report also supports caution around becoming lax on protective COVID-19 measures such as physical distancing and wearing masks, especially around vulnerable groups, he said.

Findings may have implications for health policies

“It’s also important for people who are fully vaccinated to understand that these infections are expected and are happening, especially now with the Delta variant” Dr. Hijano said. “While the outcomes are favorable, you need to still protect yourself to also protect your loved ones. You want to be very mindful that, if you are vaccinated and you get infected, you can pass it on to somebody that actually has not been vaccinated or has some of these risk factors.”

The authors of the new research paper believe their findings may have implications for health policies regarding the timing between vaccine doses, COVID-19 booster shots, and for continuing personal protective measures.

The authors of the paper and Dr. Hijano disclosed no conflicts.

Breakthrough infections twice as likely to be asymptomatic