COVID-19 vaccines are effective at reducing severe illness and hospitalization, study finds

People who tested positive for SARS-CoV-2, the virus that causes the novel coronavirus (COVID-19),  after one or two vaccine doses had significantly lower odds of severe disease or hospitalization than unvaccinated people, according to a new study investigating COVID-19 breakthrough infections published in journal The Lancet Infectious Diseases. Researchers also found that the odds of experiencing long COVID, illness lasting 28 days or more after a positive test, were cut in half for people who received two vaccines doses.

People most vulnerable to a breakthrough infection after their first vaccine dose included frail older adults 60 years and older, and older adults living with underlying conditions such as obesity, heart disease, kidney disease, and lung disease. In all age groups, people living in deprived areas, such as densely populated urban settings, were more likely to experience a breakthrough infection. These factors were most significantly associated with a post-vaccination infection after receiving the first vaccine dose and before receiving a second dose, the researchers said.

Using self-reported data from the U.K. COVID Symptom Study through the ZOE app from December 8, 2020 through July 4, 2021, researchers found that of more than 1.2 million adults who received at least one dose of either the Pfizer-BioNTech, Moderna, or Oxford-AstraZeneca vaccine, fewer than 0.5 percent reported a breakthrough infection more than 14 days after their first dose, with 6,030 positive cases after 1,240,009 first vaccine doses. Among adults who received two vaccine doses, fewer than 0.2 percent experienced a breakthrough infection more than seven days after their second dose, with 2,370 positive cases after 971,504 second vaccine doses.

For those who did experience symptoms after either one or two vaccine doses, such as fatigue, cough, fever, and loss of taste and smell, almost all symptoms were reported less frequently than in unvaccinated people. In frail adults over the age of 60 years, the odds of a breakthrough infection after one vaccine dose were almost doubled, compared to healthy older adults. Additionally, in older adults who had received their first vaccine dose but not their second, kidney disease, heart disease, and lung disease were underlying conditions associated with a breakthrough infection.

The risk of a breakthrough infection was related to the place a person lived, with people living in areas with the highest deprivation having the highest odds of a breakthrough infection following their first vaccine dose. The authors note that this could be due to many underlying causes including closer living quarters and lower overall vaccination rates in those communities, making the virus more likely to be spread.

The authors note some limitations of the study. The research uses self-reported data and therefore the reported comorbidities, test results, and vaccination status could be inaccurate or incomplete, and individuals living in more deprived areas could be underrepresented. Furthermore, the authors said that these findings might not apply to all timepoints post-vaccination, to settings with different proportions of SARS-CoV-2 variants, or to countries with different vaccination schedules.

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