Among those who did experience a breakthrough infection, the odds of it being asymptomatic increased by 63 per cent after a single vaccine dose and by 94 per cent after the second dose.
People who tested positive for SARS-CoV-2 (coronavirus) after one or two vaccine doses had significantly lower odds of severe disease or hospitalisation than unvaccinated people, according to a large-scale study investigating Covid-19 breakthrough infections published in The Lancet Infectious Diseases journal Wednesday.
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 fully vaccinated with two doses.
People most vulnerable to a breakthrough infection after a single 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 occurring in between the two doses.
Using self-reported data from the UK Covid Symptom Study through the ZOE app from December 8, 2020 to July 4, 2021, researchers found that of the more than 1.2 million adults who received at least one dose of either the BNT162b2 (Pfizer-BioNTech), ChAdOx1 nCoV-19 (Oxford-AstraZeneca), or mRNA-1273 (Moderna) vaccine, fewer than 0.5 per cent reported a breakthrough infection (6,030 positive cases after 12,40,009 first vaccine doses) more than 14 days after their first dose. Among adults who received two vaccine doses, fewer than 0.2 per cent experienced a breakthrough infection (2,370 positive cases after 9,71,504 second vaccine doses) more than seven days after their second dose.
“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. 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. Our findings highlight 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,” says study co-lead author Dr Claire Steves of King’s College London, UK.
Among those who did experience a breakthrough infection, the odds of that infection being asymptomatic increased by 63 per cent after one vaccine dose and by 94 per cent after the second dose. Researchers also found that the odds of hospitalisation were reduced by approximately 70 per cent after even partial vaccination and the odds of experiencing severe disease—defined as having five or more symptoms in the first week of illness—were lessened by approximately one-third.
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.
Age and comorbidities contribute to breakthrough infections
In partially vaccinated frail adults over the age of 60 years, the odds of a breakthrough infection were almost double that of in healthy older adults. Additionally, in partially vaccinated older adults, 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 increased risks of breakthrough infections for frail, older adults—especially those living in care homes or who require frequent visits to health care facilities—and for other people living in deprived conditions reflect what we’ve seen throughout the pandemic. These groups are at a greater risk of exposure and are therefore more vulnerable to infection. Health policies designed to prevent infections, including policies around timing between the first and second dose and potential booster shots, should prioritize these groups,” says co-author Dr Rose Penfold of King’s College, London, UK.
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