Effectiveness against all Covid infections fell from 88% within a month of having two doses to 47% after six months, according to the research. However, effectiveness against hospitalisations remained high at 90% overall – and crucially across all variants, including delta.

The findings, published in the Lancet, are consistent with preliminary reports from the US Centers for Disease Control and Prevention and Israel’s ministry of health that suggested protection against the virus fades within six months.

The results underscore the vital importance of improving Covid vaccination rates globally, researchers said.

In the UK, the NHS is already offering booster jabs to people who had their second vaccine at least six months ago, are living in residential care homes for older adults, are over 50, or are frontline health and social care workers.

It is also inviting 16- to 49-year-olds with underlying health conditions that put them at higher risk of severe symptoms and adults who have household contact with immunosuppressed individuals to go for their third inoculation.

“Our study confirms that vaccines are a critical tool for controlling the pandemic and remain highly effective in preventing severe disease and hospitalisation, including from the Delta and other variants of concern,” said the study’s lead author, Dr Sara Tartof, of the Kaiser Permanente Southern California Department of Research and Evaluation.

Dr Luis Jodar, senior vice-president and chief medical officer of Pfizer vaccines, said: “Our variant-specific analysis clearly shows that the BNT162b2 vaccine is effective against all current variants of concern, including Delta. Covid-19 infections in people who have received two vaccine doses are most likely due to waning and not caused by Delta or other variants escaping vaccine protection.”

Prof Penny Ward, visiting professor in pharmaceutical medicine at King’s College London, who was not involved in the study, said: “This publication describes real-world evidence from the US on the effectiveness of the Pfizer/BioNTech vaccine assessed using data from a large healthcare database from the Kaiser Permanente organisation.

“In general, if the objective of vaccination is to prevent illness and prevent continued spread of infection, the information suggests need for boosters six months after completion of the first vaccine course, particularly among the most vulnerable, in whom infection may lead to more severe illness and death. This approach has already been adopted in the UK, where the booster campaign is now under way.”

Meanwhile, amid reports that winter could see a rise in infections, nursing leaders are warning that nurses are experiencing more sickness, including for anxiety and depression, which could affect patient care.

The Royal College of Nursing analysed figures for staff sickness from before the pandemic and earlier this year and found thousands of days lost to staff absence on already overstretched wards.

The NHS in England recorded 73,209 more sick days among nurses and health visitors in May 2021 compared with May 2019 – a rise of 18%. Staff are now more at risk of mental health problems, chest and respiratory problems and migraines than before the pandemic, the analysis showed.

Since May 2019, the number of full-time equivalent (FTE) days lost for mental health reasons has increased by 31.4%, from 102,491 in 2019 to 134,669 in 2021.

The RCN council’s chair, Carol Popplestone, said: “There will be immense pressure on health and care services this winter, and services can’t afford to lose safety-critical professionals to avoidable illnesses on top of tens of thousands of nursing vacancies.”