Covid-19 vaccines are less effective on people with weakened immune systems, three small Italian studies show, which the studies' researchers say highlight the need to deploy booster shots for this group of vulnerable people.

Reuters has reported that the studies show that, on average, 30 per cent of immunocompromised patients do not develop immunity to the virus after vaccination.

The remaining 70pc respond to the vaccine, especially after the second dose, but to a lesser extent than healthy people and with differences from group to group, the Bambino Gesu Hospital in Rome, which conducted the three small studies, said in a statement.

The results indicate the need to increase the level of protection of the most vulnerable with booster doses, the hospital said.