How well will our vaccines hold up against Delta?

The three vaccines making up Australia’s COVID-19 strategy are effective against the Delta variant, according to a UK preprint study released this week.

Since Pfizer, AstraZeneca and Moderna were first trialled and approved, the goalposts of the global pandemic have been well and truly moved by the rise of the Delta, the variant of concern taking over across the world.

As the variant moves through populations with much higher vaccination rates than ours, researchers are getting their first look at the real-world impact of Delta on vaccine effectiveness months after many were vaccinated. The study from the University of Oxford, UK looked at how vaccines in the UK impacted Delta infections.

The researchers say two doses of either mRNA vaccine, Pfizer or Moderna, or the AstraZeneca vaccine remained the best protection from the Delta variant, but the vaccines are less effective against Delta than Alpha, the variant of concern first identified in the UK nearly a year ago.

Professor Paul Hunter from the University of East Anglia, UK told the UK SMC it was important to note this paper is focused on the ability of vaccines to prevent any infection or symptoms at all, rather than their ability to prevent hospitalisations and death.

“There is now quite a lot of evidence that all vaccines are much better at reducing the risk of severe disease than they are at reducing the risk from infection,” Prof Hunter said.

“The main value of immunisation is in reducing the risk of severe disease and death and the evidence available shows that protection lasts longer against severe disease than against mild disease. All current UK vaccines are very good at this, even against the Delta variant.”

The UK is currently using both AstraZeneca and Pfizer like Australia, as well as the Moderna vaccine recently approved in Australia and set to be rolled out in the coming weeks.

Prof Hunter said the study showed AstraZeneca to be less effective overall than Pfizer, but the effectiveness of AstraZeneca appeared to last longer. 

“Interestingly, the Moderna vaccine seems to be more effective against Delta after a single dose than either Pfizer or AstraZeneca as a single dose, or indeed after two doses of AstraZeneca,” he said.

Associate Professor Simon Clarke from the University of Reading, UK said it was concerning that AstraZeneca appeared to be no more effective at preventing Delta infection than the ‘natural’ immunity gained from a previous COVID-19 infection.

With over six million cases of COVID-19 detected in the UK since the beginning of the pandemic, the ramifications of this finding are quite different in Australia where the vast majority of the population has no natural immunity. Most will have the opportunity to be vaccinated before they come into contact with the virus as restrictions are eased. 

However, Prof Hunter said even those with natural immunity from a previous COVID-19 infection would benefit from any vaccination.

“The paper does show that immunisation after a natural infection offers quite a bit greater protection than immunisation alone or presumably natural infection alone,” he said.

With the UK vaccine rollout beginning at the end of 2020, the study was able to look at how effective vaccines remain over time. It appears both vaccines do lose effectiveness, with Pfizer’s effectiveness appearing to reduce at a faster pace than AstraZeneca’s. Because of this, the researchers estimate the effectiveness of Pfizer comes in line with that of AstraZeneca after about four to five months.

In the future, A/Prof Clarke said declining immunity could be addressed with additional doses of vaccine.

“On this evidence, it certainly supports the case for third ‘booster’ jabs for vulnerable individuals, as is now happening in Israel,” he said.

For unvaccinated people, the paper shows protection from being surrounded by vaccinated people may not be enough, with the ‘peak’ levels of Delta virus in vaccinated people who do become infected comparable to those who received no vaccine. This suggests a vaccinated COVID-19-positive person is just as infectious as an unvaccinated COVID-19-positive person at the height of their infectiousness.

While it is unclear whether the ‘peak’ of the virus lasts as long in vaccinated people compared to unvaccinated people, A/Prof Clarke said it highlighted the importance of taking precautions even after vaccinations.

“While vaccinated individuals are less likely to get COVID-19, we also see that it did not affect the amount of Delta variant which could be ‘shed’ from coughs and sneezes when someone does get infected. So, although vaccines are still reducing cases and asymptomatic infection, we still need to take seriously the signs of potential infection when fully jabbed,” he said.

Professor Stephen Evans from the London School of Hygiene & Tropical Medicine, UK said this reinforced the message that vaccinations do not eliminate COVID-19.

“More importantly, our data shows the potential for vaccinated individuals to still pass COVID-19 onto others, and the importance of testing and self-isolation to reduce transmission risk,” Prof Evans said.

Dr Penny Ward from Kings College London, UK agreed test, trace and isolate was a strategy that would need to remain even at high vaccination rates due to the ongoing risk for unvaccinated people.

“All of this reiterates the pressing need for additional options for epidemic control within the community. This might include using monoclonal antibodies with activity against the delta strain early to prevent onward transmission of infection in households, as well as early intervention with antiviral treatments both to further reduce risk of serious disease despite vaccination in an infected individual, and to prevent onwards transmission to their close contacts,” Dr Ward said.

You can read the study here and the full UK SMC expert reaction here.

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