Why the gap between initial Covid vaccines and boosters is so important 

Niac said boosters should follow 'at least five months' after initial doses. The HSE is currently offering boosters with a six-month gap.

The Covid-19 booster vaccine is to be rolled out to thousands people following emerging evidence around waning immunity and booster effectiveness.

Updated advice from the National Immunisation Advisory Committee (Niac) extends boosters to everyone aged 50 to 59, people aged 16 to 59 with an underlying condition and all residents in residential care. Up to Friday, 299,000 doses were given to over-60s, older residents in long-term care, immunocompromised patients or healthcare workers. 

Niac said boosters should follow “at least five months” after initial doses. The HSE is currently offering boosters with a six-month gap.

Dr Anne Moore, vaccinologist at University College Cork, said this gap was “really important". 

“Timing between immunisations is really important – longer is better,” she said. 

"Sometimes, a longer interval can permit even stronger responses to be induced. Giving a vaccine too soon after a previous vaccine can ameliorate its action.” 

She said there was now “documented evidence” vaccine effectiveness  decreases over time, particularly for older people, meaning the odds of someone testing positive for Covid was increasing.

“The currently authorised vaccines are really great for preventing disease, hospitalisation, death but don’t prevent infection or transmission, at least they don’t do this after a few weeks post-vaccination,” she said.

Although the gap between vaccines and boosters is based on science, that is not reassuring for thousands like Brid McArdle, 66, who completed her AstraZeneca doses on July 5.  

“The difference for me in how they do these boosters is I will have a comfortable Christmas, or I won’t,” she said. “I could have a Christmas where I have to be over-cautious because of the waning and the numbers in the community.” 

Ms McArdle, from Dundalk, said she was already wearing masks again around her school-aged grandchildren.

“I am living rather cautiously at the moment,” she said. 

"Based on the evidence around waning immunity, I would consider myself not really vaccinated at this stage.” 

Indeed, a new study led by the UK Health Security Agency stated vaccine effectiveness dropped significantly five months after vaccination. It found boosting with Pfizer vaccines gives more than 90% protection against symptomatic Covid-19 for people who had AstraZeneca or Pfizer first.

Dr Moore said there could be a need to give boosters to vulnerable people more regularly but nothing concrete had yet emerged.

“Most people in the vaccine industry would prefer to not have seasonal campaigns,” she said. “We would all like to find the most effective vaccine with the fewest number of immunisations. I don’t think we’re there with the current vaccines.” 

This comes against a background of rising pressures on the health system. The Irish Hospital Consultants Association has warned: “We have fewer ICU beds on a population basis than in 2009” despite the pandemic pressures.