Researchers also found that the odds of experiencing long COVID were reduced to half for people who received two vaccines doses
London: People infected with the SARS-CoV-2 virus after receiving one or two COVID-19 vaccine doses have significantly lower chance of severe disease or hospitalisation than unvaccinated individuals, according to a large-scale study published in The Lancet Infectious Diseases journal on Tuesday.
Researchers also found that the odds of experiencing long COVID -- illness lasting 28 days or more after a positive test -- were reduced to 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 those living with underlying conditions such as obesity, heart disease, kidney disease, and lung disease, they said.
The study found that 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-immunisation infection after receiving the first vaccine dose and before receiving a second dose, it found.
"We are at a critical point in the pandemic as we see cases rising worldwide due to the Delta variant. 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," said study co-lead author Claire Steves of King's College London, UK.
"Other research has shown a mortality rate as high as 27 per cent for hospitalised COVID-19 patients. We can greatly reduce that number by keeping people out of the hospital in the first place through vaccination," Steves said.
The study highlights the crucial role vaccines play in larger efforts to prevent COVID-19 infections, which should still include other personal protective measures such as mask-wearing, frequent testing, and social distancing.
The researchers used self-reported data from the UK COVID Symptom Study through the ZOE app from December 8, 2020 through July 4, 2021.
They found that of more than 1.2 million adults who received at least one dose of either the Pfizer-BioNTech, Oxford-AstraZeneca, or Moderna vaccine, fewer than 0.5 per cent reported a breakthrough infection over 14 days after their first dose.
Among adults who received two vaccine doses, fewer than 0.2 per cent experienced a breakthrough infection more than seven days after their second dose, the researchers said.
Among those who did experience a breakthrough infection, the odds of being asymptomatic increased by 63 per cent after one vaccine dose and by 94 per cent after the second dose, they said.
Researchers also found that the odds of hospitalisation were reduced by approximately 70 per cent after one or two doses, and that the odds of experiencing severe disease were lessened by about one-third.
Also, the odds of long COVID were reduced by 50 per cent after two doses, they said.
Severe disease was defined as having five or more symptoms in the first week of illness.
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, according to the study.
In frail adults over the age of 60 years, the researchers said, the odds of a breakthrough infection after one vaccine dose were almost doubled, compared to healthy older adults.
The authors acknowledged some limitations of the study.
The research used self-reported data and therefore the comorbidities, test results, and vaccination status could be inaccurate or incomplete, and individuals living in more deprived areas could be underrepresented.
These findings may not apply to all timepoints post-vaccination, to settings with different proportions of SARS-CoV-2 variants, or to countries with different vaccination schedules, the researchers added.