About one in 10 patients were infected with coronavirus while in hospital during the first wave of the pandemic in the UK – and that figure rose to about one in six after the peak of Covid hospital admissions, researchers have found.

However, vaccines, improvements in diagnostics and PPE supply, and an enhanced understanding of the virus have dramatically reduced the risk of catching Covid in hospital settings, they stressed. “Things have changed for the better, enormously,” said Dr Christopher Green, a senior clinical lecturer & consultant physician in infectious diseases at the University of Birmingham.

During the first wave, on average 11.3% of Covid patients in the UK contracted Covid-19 after being in hospital for other reasons, and that percentage spiked to about 15.8% in May after the peak of hospital admissions. This was the worst of the pandemic, as far as hospitals were concerned, said Green.

The average level of hospital-acquired Covid was not entirely surprising, the researchers said, given that the average risk of contracting flu in hospital settings is about 10%.

The researchers studied 72,157 patients (two-thirds of all hospital admissions in the UK) admitted across 314 hospitals during the first wave. They inferred which patients were infected after admission, based on information on how long it takes to show symptoms following infection with the virus that causes Covid-19.

Using the cut-off date of 1 August 2020, the researchers estimated that between 5,699 to 11,862 patients were infected during their hospital stay, but cautioned that this range was likely to be an underestimate given that the researchers could not identify patients who were infected during admission but were discharged before developing symptoms, or patients infected during another healthcare visit before admission.

There were the stark variations in the risk of contracting Covid in different hospital settings: acute and general care hospitals had the lowest proportion at about 9.7%; residential community care and mental health hospitals had a higher overall proportion, 61.9% and 67.5% respectively.

Given that patients in residential and long-term facilities tend to stay for protracted periods, it is not unusual that those rates were higher.

However, there were also substantial variations among acute and general hospitals. In some busy hospitals there were outstanding examples of good infection prevention control and practice, but there were also instances where the standard was not as good, said Calum Semple, a professor in child health and outbreak medicine at the University of Liverpool.

The scientists attributed the variations to multiple factors, including inadequate PPE and test supply as well as a poor understanding of how the virus transmits, but noted that these wide variations meant that there were important lessons to be learned for future outbreaks.

Although this data has now been published in the Lancet journal, the researchers have been sharing their analysis with the NHS and policymakers since last year. They continue to monitor what proportion of cases could be hospital-acquired – and that range is looking much better at 3% to 5% now, said Green.

“I don’t want people to be afraid about coming into hospitals because we do actually understand a lot more that we didn’t know back then,” he said.

“One of the key differences, of course, is the high rates of vaccine coverage – being vaccinated … dramatically reduces the risk of you getting sick – but even if you’re unlucky enough to still be infected despite being vaccinated, it also dramatically reduces the ability for you to transmit to somebody else.”

Hopefully, there will now be a greater focus on reducing the risk of hospital-acquired infections, added Semple. “Looking at hospital-acquired infections needs to be business as usual for [NHS] trusts, rather than an exceptional activity.”