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A second coronavirus vaccine booster jab may be needed within the next few years, experts say, with immunity against the virus likely to drop below 50 per cent in the six months after a second dose.

Data from the federal Health Department shows 284,000 people aged over 16 have received the booster, which is just under half the 577,000 people currently eligible for the shot. Roughly 1.6 million people in Australia will be eligible for a booster dose in December.

Analysis has found a third jab within a year more than doubled levels of immunity from two doses.Credit:Edwina Pickles

In an analysis published in the Lancet, researchers from the Kirby Institute, the Doherty Institute and the University of Sydney found that a third, booster jab within a year more than doubled levels of immunity than that seen after the first two doses.

The research also showed that boosters work at improving immunity against current variants, including Delta.

Co-author on the paper Professor Jamie Triccas, from the University of Sydney, said the research showed a booster doses are very effective, “but it is very unlikely a third dose will protect us for life”.

A senior research fellow at the Kirby Institute and another co-author on the paper, Dr Deborah Cromer, said boosters would be essential in the fight against COVID-19.

“A major implication of our research is that to keep up immune protection booster shots are needed,” she said. “Reassuringly though, protection against severe disease and death will likely remain high beyond the first year.”

“We don’t know what variants will emerge in the next couple of years, and what vaccines will be needed. The virus has continued to surprise us. But without boosters, protection from symptomatic COVID-19 could drop below 50 per cent after six months, which means more people will become infected.”

Dr Cromer said the vaccines have been super effective, but their effectiveness a year after two doses may have fallen to about 40 per cent “and will drop further after the next year”.

“Beyond that year it will keep dropping and dropping,” she said. “We will likely need a fourth dose but not for a lot longer. So, I wouldn’t be surprised if we find that it is significantly longer than the six months that was needed for the third dose before we need another one.”

Professor Stephen Turner, head of the department of microbiology at Monash University, who was not involved in the research, said we don’t yet know how long immunity lasts after a third dose.

“It might be that this is the thing that basically sets up long term immunity, maybe just two shots isn’t enough,” he said. “It might be that again, we see waning immunity where you might need another booster over time. But they’re open questions at the moment.”

Professor Turner said the report was timely as more people become eligible for boosters in Australia.

“This data really fits nicely with data that’s emerging out of Israel and UK showing that boosters really do work,” he said.

“They’ve seen that when you give boosters you restore the vaccine efficacy back up to close to 90 per cent, and particularly against severe disease.”

Pfizer is the only vaccine approved for use as a booster so far in Australia, and the medical regulator is considering an application from Moderna on the use of its vaccine as a third dose. The Commonwealth has ordered 60 million doses of Pfizer and 15 million doses of Moderna for 2022.

A spokesperson for the Health Department said that eligible aged care residents and workers will be able to access booster doses through in-reach clinics. People are eligible for a booster dose at least six months after completion of a primary course of a COVID-19 vaccine.

“Every residential aged care facility is being offered an in-reach booster clinic delivered by Commonwealth vaccine providers aligned with timing of when second doses were delivered. This program commenced on 8 November 2021,” said the spokesperson.

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