Booster doses of Pfizer could get us closer to herd immunity (Image: Adobe)

Israel has begun offering third doses of the Pfizer vaccine to its at-risk, vaccinated population. The reason? Immunity in some vaccinated people is decreasing faster than in others, with herd immunity unlikely even after 90% of a population is fully vaccinated. 

The heightened transmissibility of COVID-19’s Delta variant combined with gaps in the vaccines’ ability to fully prevent infection means variants will continue to be passed on from person to person. In Australia, this could have far-reaching implications when our borders reopen, with natural immunity low among the general population. 

While herd immunity might be off the table for at least another year, booster doses of Pfizer could get us closer to that magic number — though Australia has already fallen behind Europe and North America in securing more supplies of the Pfizer vaccine. 

The vaccine gap

The Delta variant is about 60% more transmissible than previous strains, with a reproduction number between six and seven — meaning an infected person will infect another seven people on average without social distancing precautions. 

While both Pfizer and AstraZeneca are effective at preventing serious illness and hospitalisation, people can still catch and transmit the virus, often without symptoms. 

Two doses of Pfizer is about 80% successful at stopping any Delta infection for those aged 16 to 60, while two doses of AstraZeneca is about 40% to 60% effective. Vaccinated people are 50% less likely to transmit the virus to others. 

Monash University adjunct Professor Michael Georgeff told Crikey this means that once international borders open up, with vaccinated travellers able to visit without quarantining, the virus will spread. In 2019 there were 9.4 million international arrivals. 

“Eventually it’s going to infect the number of people it’s going to infect … and it’s going to be very hard to prevent outbreaks,” he said. 

If 85% of Australians were fully vaccinated with Pfizer, Georgeff estimated about four million people would eventually become infected, with COVID-19 hospitalisation rates matching hospitalisation rates for flu and influenza. 

“Unvaccinated people are going to hospital between five and 10 times more than the vaccinated, and that’s what’s going to eventually blow the health system out of the water,” he said, adding that masks and social distancing would have to remain in place to ensure those infections are spread out over time. 

This is already happening in the US and UK where, as restrictions are eased and quarantine-free international travel opens back up for vaccinated residents, there have been spikes in cases. In the week to Sunday, confirmed cases in the US rose to 136,351 — the highest weekly rise since April 2020. 

Herd immunity unlikely 

University of Melbourne epidemiologist Professor Tony Blakely told Crikey that even if Australia reached vaccination rates of 90%, herd immunity is still likely off the table. 

“We’re not going to get there, particularly when you think viruses are going to mutate and probably become more resistant to the current vaccines,” he said. 

Blakely says that, given Australia’s contact tracing methods had improved, the country could likely open up with 70% of the population vaccinated. The Doherty Institute is expected to release modelling on reopening borders in the coming weeks.

“When the virus gets in it will pop up here but it won’t take off like crazy … and some percentage of the Australian population will get naturally infected over the next couple of years,” Blakely said. 

Fortunately, there’s emerging evidence a booster dose of Pfizer or other vaccines can protect against new strains and boost immunity as the body’s response to the vaccine wanes. But the global shortage of vaccines means this is a long way off for Australia. World Health Organization officials have called for countries to hold off on organising booster doses until other parts of the world have received doses.