Victorians who have only had two vaccinations are seven times more likely to end up in intensive care if they catch COVID than those who have had their booster shot.
The analysis of the data from the Victorian Department of Health was prepared for the Herald Sun.
It also showed that those with all three vaccinations were four and a half times less likely to end up in the hospital than those with just two shots and six times less likely than unvaccinated residents.
Speaking to the Herald Sun, deputy chief health officer Dan O'Brien, said those who have only had two shots had a "false sense of security".
"The fact is, it's not a two-dose vaccine; it's a three-dose vaccine," he said.
"With the two doses, over time, that protection reduces significantly, so three months or more out from your second dose, you're starting to really lose your protection and the benefits."
Under new advice from the Australian Technical Advisory Group on Immunisation, people aged 16 and older who have received a booster are considered "up to date" with their vaccines.
Those who got a second dose more than six months ago are "overdue". Boosters are recommended three months after a second vaccine.
Children under the age of 16 are still considered "up to date" following two vaccine doses.