Chart of the week: How has the risk of acquiring Covid-19 in hospital changed in the last year?

Each week we present analysis of data in chart form to illustrate some key issues and invite discussion. Back in November, Sarah Scobie revealed the rise in the proportion of patients in hospital who had acquired Covid-19 after being admitted. This week she revisits the topic to see if things have changed since, and debates the possible reasons why.

Chart of the week

Published: 13/08/2021

Our chart of the week last November discussed the increase in the proportion of patients in hospital who acquired Covid-19 after being admitted, defined as being diagnosed more than a week after admission. This week we revisit this topic, and examine the relationship between hospital-acquired Covid-19 and the rises and falls in cases over the past year.

The proportion of hospital patients infected with Covid in hospital increased rapidly during the second wave, reaching 24% by the second week in December. Higher transmission rates in hospital could reflect factors such as overcrowding and pressure from higher bed occupancy, as well as more staff contracting Covid in the community.

Cases of Covid-19 in the community had been rising since last September, leading to a lockdown in November. Although cases dipped following the lockdown, they began to rise rapidly during December as the more transmissible “Kent”, or Alpha, variant of Covid-19 took hold.

However, while case rates in the community continued to grow, the proportion of hospital cases where transmission occurred in hospital then declined, and has continued to do so. And when cases soared in the community in May and June, following the move out of lockdown and the spread of the Delta variant, the proportion of cases acquired in hospital remained at about 5% or below.

A key factor in the divergence of community case rates and hospital acquired cases appears to be vaccination of NHS staff. The roll-out of vaccines to health care workers began on December 8, and by January 10 up to 960,000 health and social care workers may have received their first dose.*

Other factors that may have played a part include the speed and reliability of testing patients before admission, lessons learnt from early outbreaks, and a better understanding of how transmission takes place.

A low level of hospital-acquired cases provides a safer environment for individual patients, with better outcomes and shorter length of stay. The current rate – although much reduced – still equates to 274 patients who are likely to have acquired Covid-19 in hospital in the last week of July alone.

Reducing hospital-acquired infection will also enable the NHS to operate more efficiently. Alongside testing people for Covid-19 before planned treatment, reduced transmission in hospitals could enable the NHS to recover productivity losses that have occurred as a result of Covid-19, and resume higher levels of diagnostic and planned hospital treatments.



  • *Vaccination data split by health care workers was not published until 17 February. However, by the week ending 10 January there were 960,699 people under 80 who were vaccinated. Only people aged 80+ were able to be vaccinated in the general population at the time.  
  • This definition of hospital-acquired infection may include some patients who caught Covid-19 in the community, but had an incubation period of eight days or more. The definition will miss cases where the patient acquired Covid-19 in hospital but the incubation period was less than eight days, or where they were not diagnosed until after they left hospital.