New research has revealed that the Pfizer vaccine may not be as effective on older adults as it is on younger recipients.

Researchers from the University of Virginia, USA, tested the blood of 167 people who received Pfizer or Moderna vaccines and found Pfizer recipients aged 50 and over had fewer antibodies in their system than those aged under 50. The researchers found no age-related difference in antibody levels for Moderna recipients, and say antibody levels overall were greater for Moderna recipients than Pfizer.

Antibodies word to bind to the virus and neutralise it, preventing it from infecting cells in the body. The more antibodies a body has, the more effective it is at eliminating the virus. The study results were published in medical journal JAMA.

The findings come as the Australian government ramps up its campaign to get older Australians vaccinated, with many over-50s hesitant to receive the AstraZeneca jab over fears it could lead to a rare blood-clotting disease. This week it was revealed that Prime Minister Scott Morrison will send a letter to over-60s urging them to get vaccinated as soon as possible.

In June, an Australian Bureau of Statistics survey found that thousands of older Australians were delaying getting vaccinated with AstraZeneca in the hopes they could get Pfizer when it became more readily available across the country.

The ABS surveyed unvaccinated people across different age groups on their reasons for not getting a jab. A third of respondents aged between 50 and 69 and one in four respondents aged over 70 said they were waiting for a “different vaccine”.

AstraZeneca is currently only recommended for adults aged 60 and over. The government raised the recommended age for AstraZeneca in June after it was revealed the vaccine could cause extremely rare but potentially deadly blood clots.

News that a small number of older people had died from the blood clotting condition after receiving AstraZeneca spurned concerns among Australia’s over-60s, despite health experts warning that the risk of dying from Covid-19 was far greater than the risk of side effects or death from the jab.

Vaccine hesitancy, as well as low vaccine supply, has hampered Australia’s jab rollout. Currently only 36.4 per cent of Australians aged over 16 are fully vaccinated — well short of the government’s 70 per cent target to open the country again.

AstraZeneca is also set to be phased out of Australia’s vaccine rollout from October.

The government unveiled an updated six-month plan for the vaccine rollout in June, outlining how it intended to direct supplies over the rest of the year. Modelling predicts that by October, there will be between 1.7 million and 2.3 million doses of Pfizer available nationwide each week, while a minimum of 430,000 Moderna doses are forecast to be handed out between October and December. Moderna should be available in Australia from September.