People that were given the Janssen COVID-19 vaccine can get a booster dose after just three months, under new recommendations.
The Chief Medical Officer has endorsed the recommendations from the National Immunisation Advisory Committee in his latest letter to the Health Minister.
Most of the more than 235,000 people who received the Janssen (J&J) shot did so in the spring or early summer.
Earlier this week, NIAC recommended booster doses for everyone over the age of 50 and people aged 16 to 59 who are living with an underlying condition.
Third doses were already being rolled out to the over-70s, health workers and the immunocompromised.
It means people in those cohorts who received the J&J shot are now eligible for a booster after three months.
Those who received any of the other three jabs have to wait until at least five months after they were deemed fully vaccinated (one week after second shot for Pfizer and two weeks after second shot for Moderna).
The letter notes that international data suggests vaccine effectiveness wanes quicker for older people, the immunocompromised and those with underlying conditions.
It points out that 81% of the COVID patients over the age of 15 who were admitted to ICU in Ireland between June 27th and October 30th had an underlying condition.
NIAC is recommending a full dose of Pfizer or a half dose of Moderna as a booster shot; however, it has warned that Moderna should not be offered to people under the age of 30 as a precaution due to preliminary data indicating an increased frequency of myocarditis among that age group.
Dr Holohan warns that there is no data on the long-term effectiveness of booster doses – but notes that real-world evidence from Israel shows boosters increase the defence against infection and severe disease in the short term.