However, today's study shows that the age of a patient did not appear to affect their risk of passing on infection, despite suspicions that children may be more infectious than adults.

Today's research also suggests that most transmissions occur shortly before or after the first patient shows symptoms of infection. It shows that the risk of someone catching the virus is higher in households of only two people compared to households of six people: 28% of household contacts developed acute respiratory illness in households of two people, compared to 9% in households of six people. The authors of the study believe this is because in larger households there is less one-on-one contact between family members.

The results show that one in eight of the 600 people living with swine flu patients developed symptoms of respiratory illness. This figure is in the lower range of values of what was observed in past flu pandemics.

Finally, today's study shows that no particular symptoms, including cough, runny nose, fever, sore throat, vomiting and diarrhoea, were more associated with the virus being transmitted between people in the same household than the others. However, for some of the symptoms, there was little power to detect an effect since, for example, almost all patients (92%) had a cough.

Source: Imperial College London