For this study, researchers at King’s College London utilized data from the COVID Symptom Study app, a free mobile app developed by health company ZOE.
This app allows Brits to track their Covid symptoms on a daily basis, while also logging test results, vaccination status, and other health information.
Over 1.2 million app users reported getting vaccinated between December 2020 – when vaccines were first made available in the UK – and July 2021.
Out of those 1.2 million, the researchers found, about 6,000 tested positive for Covid two weeks after their first vaccine dose (but before their second dose).
A smaller number – about 2,400 – tested positive at least one week after their second shot.
In other words, just 0.2 percent of fully vaccinated individuals later got a Covid infection – demonstrating how well the vaccines work.
The researchers compared these app users who had breakthrough cases to both vaccinated users who didn’t test positive for Covid and unvaccinated users who got non-breakthrough infections.
In comparing breakthrough and non-breakthrough cases, the researchers found that Covid tended to be a milder experience for those who had gotten their shots.
‘Almost all symptoms were reported less frequently in infected vaccinated individuals than in infected unvaccinated individuals,’ the researchers wrote in their paper.
These common symptoms include loss of smell, cough, fever, headache, and fatigue.
They also noted that vaccinated individuals were more likely to have an asymptomatic case – while unvaccinated patients were more likely to require hospitalization.
Vaccinated people who got breakthrough cases were also less likely to show signs of long Covid – which the researchers defined as symptoms for 28 days or more.
Specifically, fully vaccinated individuals who had breakthrough cases were about half as likely as unvaccinated individuals to report long-term symptoms.
This lowered risk of long Covid after a breakthrough case is compounded by the overall lowered risk of getting Covid after vaccination at all.
The UK researchers also analyzed demographic factors of the breakthrough case group.
They found that older adults were more likely to have a breakthrough case than younger adults.
Older adults themselves were more at risk if they had other vulnerable conditions.
Also at risk: people living in low-income areas and those with obesity.
This study relies on health information that patients report themselves, meaning the findings should be taken with a grain of salt.
The researchers noted that COVID Symptom Study app users in their analysis tended to be women, and to live in high-income areas – potentially skewing the findings.
Still, NIH director Collins said that these findings ‘offer encouraging news’ in commentary on the NIH Director’s Blog.
‘These latest findings offer the encouraging news that help is already here in the form of vaccines, which provide a very effective way to protect against COVID-19 and greatly reduce the odds of Long COVID if you do get sick,’ he wrote.
To further study long Covid – including post-vaccination risks – the NIH has launched the RECOVER Initiative, which will study tens of thousands of Covid survivors.
This initiative was announced in February 2021 and begun awarding grants to researchers this summer.