COVID-19 vaccine booster shots will be needed, but the initial two jabs of Pfizer or AstraZeneca will provide good protection despite waning immunity, a Newcastle viral immunologist says.
"With the data coming out of the UK and Israel, we know that your protection is waning from about six months," said Associate Professor Nathan Bartlett, of HMRI and the University of Newcastle.
"And so it's going to allow breakthrough infections [people becoming infected after vaccination]."
Dr Bartlett said the double jabs would still protect people from severe disease, hospitalisation and death, "even as your immunity wanes".
This is because the vaccines "start off with such a high level of protection".
"These vaccines are really good," he said.
They offer protection of 80 to 90 per cent, so "you can afford to drop 10 to 20 per cent and still have a pretty good vaccine that will protect most of us from getting ill".
"But what it is going to allow is rates of infection to creep up."
The longer waning immunity lasts in the community, the more the virus will circulate and the more breakthrough infections will occur.
"It means the virus will be circulating more and more, which is exactly what we're seeing in Israel," he said.
"The vast majority of people aren't getting super sick and going to hospital. But it does expose the vulnerable and those who aren't vaccinated or who don't get good protection from the vaccine. They'll be at greater risk.
"That's the main equation initially. It becomes a trade off."
A University of Pennsylvania study of the Pfizer and Moderna mRNA vaccines found that the immune system has a backup plan that doesn't rely on boosters.
While antibodies against SARS-CoV-2 [the virus that causes COVID-19] wane over time, the vaccines generate immune memory through B-cells and T-cells.
These cells increase over time to help guard against serious illness.
Booster shots - so-called third doses - would boost antibodies and block SARS-CoV-2 for longer.
But the findings about the B-cells and T-cells, mean the body has its own support system to defend against COVID-19 as antibody levels decline.
Dr Bartlett said the study's conclusion was "entirely reasonable and to be expected".
"That's why leaving the gap between doses is turning out to be better because you allow more time for those B- and T-cells to become established memory cells.
"Boosting them later works better than boosting them too early."
Pfizer shots, for example, are being given three weeks apart, but "it's looking now like you actually get better protection from Pfizer if you leave it six weeks".
"It works perfectly well at three weeks, but it works even better at six weeks."
Research into the AstraZeneca vaccine was "very clear that 12 weeks between shots is better than four weeks".
"But again, they're working so well even four weeks is going to give you pretty good protection."
Furthermore, people have different immune responses to vaccines.
"Everyone's immune system is unique," Dr Bartlett said.
"In a normally distributed range of responses, some people might have good antibodies for years. Some people might lose them by three months."
The notion of waning immunity around six months refers to a mean or median statistic.
"You can start to see immunity waning as early as four to five months, and by six months it's quite noticeable," he said.
"If you're starting at 89 per cent protection and dropping to something lower - like 75 per cent - you will still be very well protected."
People who had their vaccines at the start of the rollout would require boosters from September/October, if six months was used as the benchmark.
However, Prime Minister Scott Morrison has said booster doses are not planned until next year.
The Morrison government secured an additional 85 million doses of the Pfizer vaccine for booster doses.
The government had also ordered 15 million booster doses of the Moderna vaccine for the first half of next year.
"Every Australian will have access to a booster shot if it is needed," he said.
Mr Morrison is facing pressure to bring boosters forward, and vaccine supply is expected to be stronger towards the year's end.
A new COVID-19 strain named C.1.2 has been making headlines, amid fears about its potential.
This new variant comes from South Africa.
The World Health Organisation has not listed it as a "variant of concern" or "variant of interest".
The National Institute for Communicable Diseases in South Africa did issue an alert about the "C.1.2 lineage".
"This lineage possesses mutations within the genome that have been seen in other SARS-CoV-2 variants of interest or variants of concern, but also other mutations which are novel."
However, the Delta strain remains the dominant variant in South Africa and the world.
Dr Bartlett said the C.1.2 strain wasn't necessarily something to panic about.
"While Delta is so successful and dominant, it's going to be very difficult for another virus to displace it," Dr Bartlett said.
"The reason why Delta was able to displace the previous dominant virus - the Alpha virus from the UK - was that it was considerably more transmissible than Alpha.
"We're talking multiple times more transmissible."
He said the C.1.2 variant would "have to transmit substantially better than Delta to displace it".
"Delta already has an RO [reproduction number] of five. So for every one person who gets infected, they'll infect five people."
The original Wuhan virus had an RO of about two.
"It's hard to imagine mutations that would boost its transmissibility so far beyond Delta again. I'm not saying it's not possible, this virus keeps surprising us, but it's a big ask from an evolutionary point of view while Delta has such a firm grip."
When one virus is dominant, it usually excludes other viruses.
"We see that with influenza and other respiratory viruses," he said.
Hamish McCallum, an infectious disease ecologist at Griffith University, has written that the pandemic is an example of "evolution in action".
He wrote in The Conversation that evolutionary theory suggested that viral variants that can evade vaccines have "an evolutionary advantage".
"So we can expect an arms race between vaccine developers and the virus, with vaccines trying to play catch up with viral evolution.
"This is why we're likely to see us having regular booster shots, designed to overcome these new variants, just like we see with flu."
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