Giving care home staff flu jabs can cut the number of illnesses and deaths among residents, a study has shown.
The NHS offers flu jabs to the elderly and those who are most at risk
The practice makes a significant difference when flu rates are high, researchers from University College London said.
Writing in the British Medical Journal, they said staff would be the main way flu was introduced into care homes.
A Department of Health spokesman said he hoped the study would encourage homes to offer the vaccine to staff.
Elderly people living in nursing and residential homes are likely to have weakened immune systems, leaving them vulnerable to flu outbreaks even when they have been vaccinated.
Many countries offer flu jabs to healthcare workers every year, but in the UK most care homes do not vaccinate their staff.
The research, funded by the Department of Health, looked at 44 UK care homes during the winters of 2003-4 and 2004-5.
Vaccination was promoted and offered to full-time staff in half of the homes, but not the rest.
There was a high level of vaccinations among residents in all homes in both years.
Vaccine coverage among full-time staff in homes where it was promoted was 48% in 2003-4 and 43% in 2004-5 compared with 6% and 3.5% in the others.
In the 2003-4 influenza season, levels of illness and death were significantly lower in vaccination homes.
Deaths were reduced by five per 100 residents, and illness by nine cases per 100 compared to unvaccinated homes, the researchers found.
Consultations with family doctors and hospital admissions were also substantially lower in the homes where the flu jab was promoted.
But no major differences were seen in 2004-5, when national flu rates were far below average.
'Demonstrates the benefits'
Dr Andrew Hayward, who led the study, said: "I certainly think that this study shows that care home employees should be offered the vaccine.
"Nursing homes are relatively closed environments and staff are likely to be the main introducers of flu into these homes.
"Preventing flu in the staff can therefore substantially reduce exposure of residents to flu."
He said hospitals, which the study did not consider, were different environments to care homes - because more people came and went, including patients.
But he added: "Nevertheless, it is highly plausible that preventing flu in NHS staff will prevent flu and its complications in patients."
Dr Douglas Fleming of the Royal College of GPs Birmingham Research Unit, said: "Influenza spreads rapidly among frail, elderly and sick people.
"There are two good reasons for nurses and healthcare staff to be vaccinated against influenza.
"Firstly, to protect themselves from getting influenza and secondly, to reduce the likelihood that they will pass on the influenza virus to their patients.
"This study demonstrates some of the benefits that persons in care homes receive if the staff are vaccinated."