Fully vaccinated people who caught COVID-19 were twice as likely to get no symptoms, a study found.
But most vaccinated people in the study didn't catch COVID-19.
Study authors said vaccinated people who interacted with vulnerable people should get regular tests.
The study from researchers at King's College London was published in The Lancet Infectious Diseases medical journal late Wednesday. It found that fewer than 0.2% of 971,504 fully vaccinated adults studied caught COVID-19 at least a week after their second dose.
But, for the 2,370 who did get infected post-vaccination, the likelihood of having no symptoms increased by 94% compared to someone who's unvaccinated, the study authors said.
The scientists also found that the likelihood of catching COVID-19 with five or more symptoms in the first week of illness was reduced by about a third in those fully vaccinated. The likelihood of getting hospitalized with COVID-19 after two doses of a COVID-19 shot was reduced by two-thirds, they said.
The study authors said that the findings highlighted the importance of regularly testing vaccinated people regardless of symptoms. This was especially the case if they interacted with unvaccinated people or those vulnerable to COVID-19, such as those who are older or with underlying health conditions.
Dr. Claire Steves, co-lead study author, said in a statement that the 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."
The research, which was funded by the UK government, used the Zoe app, a symptom-tracker with more than 4.7 million users worldwide.
The researchers compared those who said they were vaccinated with either Pfizer, Moderna, or AstraZeneca's vaccines and those who were unvaccinated. They took into account other factors like age, health risk factors and geographical location that may impact the results.
The symptoms they looked at included: fever, chills, persistent cough, fatigue, shortness of breath, loss of smell, hoarse voice, chest pain, tummy pain, diarrhoea, confusion, eye discomfort, dizziness, sore throat, unusual muscle aches, blisters on feet, worse hayfever, hair loss, and brain fog.
The self-reported nature of the data means that some data was inaccurate or missing. A COVID-19 infection was proven either with a lab test or quicker lateral flow test.
Protection against variants
The study, which took place between December 8, 2020, and July 4, captured infections caused by both the formerly dominant Alpha variant and the highly infectious Delta variant, which has mutations that help it avoid the immune response. But the variant that the participants caught was not looked at in the study.
Previous real-world studies have shown that Pfizer and AstraZeneca's vaccines are 88% and 60% effective respectively against COVID-19 with symptoms caused by the Delta variant that's now most common in most countries worldwide, including the UK and US.
Dr. Sara Oliver, epidemic intelligence service officer at the CDC, said in a presentation on Monday that since Delta appeared, vaccine effectiveness against hospitalization ranged from 75% to 95%.
In the US, 52.4% of people are fully vaccinated, while in the UK, 64% of Brits are fully vaccinated, according to the Johns Hopkins University.
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