When Premier Mark McGowan cancelled Western Australia's February 5 border reopening he cited the Omicron variant and low rates of booster coverage as the reasons for the decision.
Only 27 per cent of residents aged over 16 have had a third jab.
"So far, the science shows that people with only two doses of a COVID vaccine have only a four per cent protection against being infected by the Omicron variant," Mr McGowan said.
"With a third dose, it can provide a 64 per cent protection against infection.
"In addition, protection against severe disease is maintained at 80 per cent to 90 per cent for people with two doses, but increases to 98 per cent for people with three doses.
How long can you hold out?
Deakin University epidemiology chair Catherine Bennett told Stan Shaw on ABC Radio Perth that although the Premier's figures were correct, Omicron was already spreading in Perth.
"In Western Australia you've been able to keep the viral levels very low, with very few local transmissions," Professor Bennett said.
"In fact, in the last week, you've seen 20 per cent of your total community transmissions [for] all time.
"But equally, while the number of people [in WA] who had their first dose is really high, there's still 10 per cent of those people waiting to have the second dose.
"And then you have wait again until you have a booster so that you get that extra effect [from the third jab]."
Mr McGowan has indicated he would like WA to reach an 80 to 90 per cent booster rate.
But with people who have yet to get their second jab still months away from getting a third, the question, according to Professor Bennett, is "how long you can hold out" until Omicron spreads further.
"You've got Omicron circulating in the community, whether the choice is really up to the state or whether Omicron itself is going to determine what the timelines might actually look like," she said.
"You already have Omicron in the community.
"The epidemiological concern with Omicron is if you have enough people who aren't vaccinated, or who are vaccinated with a primary course, but don't yet have the booster, we have seen it spread, and it's mainly in 20 to 39-year-olds.
"That's what we're seeing consistently in every state in every country, and unfortunately that's the group that have the lower rates in WA as well, in terms of their completion of the primary course."
Omicron may determine timeline
Restrictions like mask-wearing, density limits and closed borders may work to slow down Omicron's spread in WA, but Professor Bennett said it remained to be seen how effective the measures would be.
"Then the question is what role the border plays in this," she said.
"That just might make the border decision less important, if you actually have a sizeable outbreak you're trying to manage and then you're pulling all those other things in to make it work.
"If you had zero Omicron you might be in a different position.
How long will the boosters last?
It is too early to know how quickly immunity generated by a booster declines, Dr Bennett says.
"The first fourth booster doses went out in countries like Israel to the people who are most at risk of immunity waning anyway — people who are immunosuppressed or people who are elderly and don't have a stronger immune reaction," she said.
"For the rest of the population, young healthy adults, it may be enough and in fact, we have higher infection rates in Australia now, so that actually boosts that immune coverage that we have from the vaccine as well.
"At a population level, that's starting to change our immunity profile too and it might mean we don't need another booster or it might mean we do need something if a new variant comes along."
Reasons for optimism
Professor Bennett said despite the extremely high case numbers seen in the rest of Australia, the pandemic situation was very different from two years ago because vaccines had proved highly effective at protecting people from serious illness.
"The sorts of numbers we have in the communities today, across all states, including WA's numbers, which we hadn't seen before, they are thankfully not putting that same proportion of people in hospital," she said.
"You just could not cope with the sheer number of cases we have in the eastern states if they were all translating to hospitalisations in the way they were in the past.
"Our treatments still work for people who do still become very unwell and we've just seen those antivirals approved in Australia … that help to avoid disease progression in people who are very vulnerable."
