Coronavirus vaccines could be made compulsory for care home staff, the government has announced.
The Department of Health has launched a "consultation" on making it a condition of employment.
Health secretary Matt Hancock said "we have a duty of care to those most vulnerable to COVID-19".
However, Unison, the union representing public service workers, said it was the "wrong approach" and "could backfire badly".
The latest NHS statistics for older-adult care homes in England show one in five staff had not been given a first dose of the vaccine as of 4 April.
In one London borough, Lambeth, only half of eligible staff in older-adult care homes had received a jab.
Wednesday's consultation announcement comes after Boris Johnson told MPs last month it is "wholly responsible for care home companies to think of requiring vaccination".
A few days later, England's chief medical officer Prof Chris Whitty also stressed the "professional responsibility" of health and care staff to get vaccinated.
Frontline health and social care workers were second on the Joint Committee on Vaccination and Immunisation's priority list for getting a vaccine.
The government said on Wednesday that the consultation would take into account any potential impact it could have on staffing and safety, how it could be implemented and who could be exempt.
Watch: Matt Hancock discussing compulsory COVID vaccines for care home staff on 23 March
A decision is set to be made in the summer, with staff, care providers, care home residents and their families being urged to take part.
Announcing the consultation, Hancock said: “Older people living in care homes are most at risk of suffering serious consequences of COVID-19 and we have seen the grave effects the virus has had on this group.
“Making vaccines a condition of deployment is something many care homes have called for, to help them provide greater protection for staff and residents in older people’s care homes, and so save lives.
“The vaccine is already preventing deaths and is our route out of this pandemic. We have a duty of care to those most vulnerable to COVID-19, so it is right we consider all options to keep people safe.”
Responding to the announcement, Unison general secretary Christina McAnea said "boosting the number of vaccinations in the social care sector is essential for everyone’s safety" but that "mandatory jabs are the wrong approach and a massive distraction".
McAnea said the government should instead "target adverts" at staff, encourage already-vaccinated colleagues to offer reassurance and "give staff time" to make their own decision.
She added: “Too heavy-handed an approach could backfire badly. Some staff may simply up and go, leaving a poorly paid sector already struggling with thousands and thousands of vacancies in a terrible state.
"That could damage the quality of care for the elderly and vulnerable, and no one wants that.”
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