New modelling highlights the significant role booster vaccines have in dampening the transmission of the Omicron variant.
The modelling from Leighton Watson at the University of Canterbury, published as part of a report in the New Zealand Medical Journal, considered the probability of Omicron infection based on the different vaccination statuses of the infecting and infected individuals.
Using a scenario where there is an equal number of people who unvaccinated, vaccinated with two doses and boosted, the modelling found 46 percent of infections are the result of unvaccinated people. This is compared to vaccinated people being responsible for 39 percent of cases and boosted people leading to 15 percent of cases.
Of the new infections created in this model, 44 percent are unvaccinated, 38 percent are vaccinated and 18 percent are boosted.
"Unvaccinated individuals are 3.1x more likely to infect others and 2.4x more likely to be infected than boosted individuals," the study says.
Transmission between boosted individuals is only responsible for 3 percent of infections, the model found.
"These results are sensitive to the model assumptions about vaccine effectiveness, but nonetheless show that even though the Pfizer-BioNTech vaccine is less effective against Omicron than Delta (particularly for two doses), boosted individuals are much less likely to spread or be infected with COVID-19.
"Vaccinated individuals are only slightly less likely to spread COVID-19 or get infected when compared to unvaccinated individuals, demonstrating the need to get boosted to protect against Omicron infections and prevent onward transmission."
The study suggested that while the COVID-19 Protection Framework currently distinguishes between the vaccinated and unvaccinated, it should also distinguish between those who have had two doses and those who have had three.
Vaccine passes are a key part of the current framework, better known as the traffic light system. If vaccine passes are used, indoor gatherings can include 200 people, compared to just 10 if they aren't in use. Anyone who has had two doses can receive a pass.
The Government has suggested this could be updated in the future to require three doses, but is about to ditch the mandatory vaccine pass system altogether. On Monday night, unvaccinated individuals will be able to enter hospitality sites, events and gatherings as vaccinated individuals currently can.
A scenario was modelled where 10 percent of people are unvaccinated, 40 percent are vaccinated with two doses and 50 percent are boosted.
It found unvaccinated individuals are responsible for 17 percent of infections, compared to 27 percent caused by boosted people. Of the new infections, 15 percent are unvaccinated people compared to 32 percent boosted.
"The boosted population is 5x larger than the number of unvaccinated individuals, but only responsible for causing 1.6x as many infections and receiving 2.1x as many (much less than the 5x as many that would be expected if the vaccine did not offer any protection).
"This illustrates that three doses of the Pfizer-BioNTech vaccine can be effective at preventing infection and transmission."
When looking at the Delta variant, the modelling used a vaccination rate of 50 percent, meaning an equal number of unvaccinated and vaccinated individuals in the population.
The results showed that in this scenario, unvaccinated individuals are responsible for 87 percent of infections with 67 percent of all infections between unvaccinated individuals. In contrast, just 13 percent of infections are from vaccinated individuals and only 3 percent are from vaccinated to vaccinated.
If the vaccination rate was 78.7 percent of the total population, which the study says is about 90 percent of the 12+ population, unvaccinated individuals are responsible for 65 percent of infections despite only making up 21.3 percent of the population.
Again, vaccinated individuals are responsible for far fewer infections, just 35 percent. A vaccinated person has a 65 percent chance of being infected by an unvaccinated person, compared to a 35 percent chance of being infected by a vaccinated person.
"Even at high levels of vaccination, unvaccinated individuals are the main cause for continued spread of COVID-19 with only 18 percent of infections from vaccinated to vaccinated," the study says.
"This suggests that restricting unvaccinated individuals from high-risk locations (ie potential super-spreader events) will help to minimise the spread of COVID-19, which is the goal of the COVID-19 Protection Framework."
Modelling also considered what the potential number of infections in an outbreak seeded by an unvaccinated individual would be compared to one seeded by a two-dose vaccinated individual for Delta and a boosted individual for Omicron.
With Delta, the modelling found unvaccinated individuals have a 54 percent chance of causing an outbreak with more than 107 cases after 31 days. In comparison, for a vaccinated seed infection, there is a 54 percent chance the outbreak doesn't spread beyond the initial seed.
For Omicron, the modelling found little difference between unvaccinated and vaccinated seed infections, but boosted individuals "are slightly more likely to result in small outbreaks that self-extinguish".
"Increasing booster coverage can significantly slow the growth of an outbreak, making it much more likely that outbreaks will self-extinguish after a small number of cases."
