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Initially two COVID-19 vaccinations were recommended, then came the booster and now there is talk of a fourth. At what point does our immunity wane, why do we need to keep having the vaccines and do they protect us against variants?

According to Professor Peter Richmond, Head of Paediatrics at The University of Western Australia’s School of Medicine and Head of the Vaccine Trials Group at the Telethon Kids Institute, when dealing with the pandemic the idea is to get population immunity as quickly as possible.

“That generally involves giving two doses – the first one to prime the immune system and the second one to increase that level of immunity – and you want that done as quickly as possible which is generally three to four weeks apart,” Professor Richmond said.

Even though the vaccines have been effective against COVID-19, they were initially designed for the original strain of the virus.

“As we have seen new variants develop, we have seen that there is increasing evidence of waning immunity after these first two doses of the mRNA vaccines or the viral vector vaccines such as AstraZeneca,” Professor Richmond said.

“However, it is important to note that protection against hospitalisation and serious disease and death has actually been maintained even though you are still getting breakthrough infections. So the vaccines are still protecting you against severe disease.”

Booster vaccines were initially given against the Delta strain and provided very good protection after about a week of being administered, including in the highest-risk populations such as the elderly. There was little evidence of waning immunity after the booster in the first three months. But there is a significant difference between the Delta strain and Omicron.

“We have found with Omicron that waning of immunity is more rapid – it starts after about three months,” Professor Richmond said.

“That’s raised this issue that maybe we need to have a vaccine that’s designed specifically against the variant.”

People who have had three doses of the vaccine are much less likely to spread COVID to people in close contact, but waning immunity means a fourth dose is being considered – particularly for the elderly, those with comorbid conditions, immunocompromised people and potentially also healthcare workers.

“I think we would probably be talking about the second half of this year if we have Omicron-specific vaccines or maybe just a fourth of the current vaccines which could be given before winter,” he said.

“That will probably be guided the amount of COVID in places such as Sydney and Melbourne because we’re still going through our first wave, they have already gone through their Omicron wave.”

A third dose for children and teenagers is also being considered with early data suggesting that, like young adults, the waning of immunity also occurs in teenagers and in younger primary school children.

“They are at very low risk of getting infected and ending up in hospital but there is still some risk,” he said.

Professor Richmond is currently working on a which looks at the effectiveness of different types of vaccines and whether there is an advantage to mixing booster shots.

“One of the things that we are looking at in our medical research ‘PICOBOO’ trial will be duration of immunity after a third dose and we have also applied for funding to look at a fourth dose of vaccine for the at-risk population,” he said.