Happily married people are more likely to fight off flu effectively, research suggests.
Antibodies can fight off infection
Conversely, the virus may be more difficult for those who are recently bereaved or divorced.
A team from the University of Birmingham found that stressful life events had an effect on the immune response to the annual flu jab.
This immune response is thought to be a good indication of the body's capacity to fight back against the virus itself.
Flu is estimated to kill several thousand people in the UK each year
10-15% of the population develop flu each year
100,000 flu particles can be projected into the air with just one sneeze
In 12 hours, the flu virus can invade 1 million nose and throat cells
The Birmingham team examined levels of antibodies - produced by the body to combat disease - in the blood.
A higher increase in antibody levels indicates that the body's immune system is better primed to fight off infection.
People who said they were happily married had much higher levels of antibodies in the blood than those who reported lower marital satisfaction.
Those that had suffered a bereavement in the year prior to vaccination had a poorer response than those who had not suffered bereavement.
More than 180 people aged over 65 from surgeries across Birmingham took part in the study.
Participants gave a blood sample prior to vaccination, then further samples at one month.
They also completed questionnaires to gauge exposure to stressful life events.
Lead researcher Dr Anna Phillips said: "We know that those aged over 65 are more at risk of the impact of flu.
"But this research shows that within that group, those that have been recently bereaved, or those that are single, divorced or widowed are more at risk that those who are in a happy marriage.
"It is especially important for these at risk groups to get their flu jabs.
"We would like to take this research further, to see whether interventions such as bereavement counselling or marriage counselling can improve the immune response in at risk groups."
Dr John Moore-Gillon, president of the British Lung Foundation, said: "Many things can affect the immune system, and this research suggests that the state of someone's mind may be one of them.
"It's always difficult to completely exclude the possibility that it's actually factors like subtle alterations in nutrition which are responsible for the differences seen in the immune responses.
"But the research certainly shows that we need to try and understand more about how the mind and the body interact in both health and disease."