Receiving two doses of the Covid-19 vaccine reduces the risk of hospitalisation by more than two thirds (70%) and halves the odds of Long Covid, according to a study published in The Lancet Infectious Diseases journal.

Although the vaccines did not always prevent people contracting the virus, breakthrough infections were almost twice as likely to be completely asymptomatic compared with unvaccinated people.

Co-lead author of the study, Dr Claire Steves of King’s College London, said that the study is evidence that “these vaccines are doing exactly what they were designed to do – save lives and prevent serious illnesses.”

The odds of having severe disease post vaccination lessened by around one-third

The study used self-reported data from the UK Covid Symptom Study through the ZOE app from 8 December to 4 July 2021. Of the 1.2 million adults who received at least one dose of either the Pfizer, AstraZeneca or Moderna vaccine, fewer than 0.5% reported a breakthrough infection after 14 days of their first dose. This figure shrunk to just 0.2% after receiving two vaccine doses.

Of those who did experience a breakthrough infection, the odds of that infection being asymptomatic increased by 63% after one vaccine dose and by 94% after the second dose.

Researchers also found that the odds of experiencing severe disease (defined as having five or more symptoms in the first week of illness) were lessened by around one-third.

For those who did experience symptoms, such as fatigue, cough, fever, and loss of taste and smell, almost all symptoms were reported less frequently than in unvaccinated people.

People most vulnerable to a breakthrough infection after their first vaccine dose included frail, older adults (aged 60 and over), and older adults with underlying health conditions such as obesity, heart disease and lung disease.

Those living in more deprived areas, such as urban, densely populated settings, were also more likely to experience a breakthrough infection. However, these factors were more strongly associated with breakthrough infections after the first dose compared with the second.

Health policies designed to protect frail, older adults are needed

Dr Rose Penfold of King’s College, co-author of the study, said that the risk of breakthrough infections for older adults reflects the need to prioritise health policies designed to prevent infection in these groups.

“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,” she said.

Dr Steves added that the results highlight that there is still a need for other personal protective measures, such as mask-wearing, frequent testing and social distancing.