COVID-19 vaccine efficacy fades over time: Study

Correction: This article was updated at 1.00 pm Friday 5 November to clarify that the 22% decrease in Pfizer’s vaccine efficacy for every 30 days after the second dose is a relative reduction.

A pre-print study investigating how well COVID-19 vaccines protect against infection shows they are effective against Delta, but less so as time goes on.
 
The analysis, a collaboration led by researchers at the University of Oxford, assessed the rate of infection in randomly selected private households across the UK.
 
It showed that while Pfizer offers higher protection following a second dose compared to AstraZeneca – defined by measuring the viral burden detected via an RT-PCR diagnostic test – its protection fades faster.
 
The research also suggests that the Delta strain causes higher viral burdens in those infected post-vaccination than the previous Alpha variant.
 
However, while the results indicate that efficacy may drop substantially as time passes – in Pfizer’s case by as much as a 22% relative reduction for every 30 days after the second dose – the authors wrote that they do not suggest any conclusive requirement for follow-up booster injections.
 
‘Whilst protection against hospitalisation and death is maintained, “booster” vaccinations may not be needed, particularly since infection post vaccination may provide a natural antibody boost,’ the authors write.

‘However, declines in immunity against infection demonstrate this needs to be monitored closely.’
 
Peter Collignon, an infectious diseases professor at the Australian National University Medical School, believes the results reinforce the underlying assumption that the vaccines are effective.
 
‘[They] show the vaccines work; they also show you may get some waning of immunity,’ he told newsGP.
 
‘Every vaccine fades over time, that’s not a real surprise. To my mind, the main point of the vaccines is to stop you dying and to stop you getting seriously ill as judged by hospitalisation.’
 
Both vaccines reached roughly the same efficacy rates in protecting against infection from Delta after around 4–5 months, the study authors conclude.
 
Those with signs of previous COVID-19 infection showed the strongest resistance after being vaccinated, the analysis showed – a finding that could spur those who have previously had the virus to get vaccinated.
 
The need for boosters has been carefully scrutinised recently as more countries follow the lead of Israel, the first nation to begin administering a third injection to the wider population.
 
The US Centers for Disease Control and Prevention (CDC) also recently revealed plans for a booster program, although the announcement did not cite concrete evidence that waning immunity will result in people being more at risk of chronic illness.
 
‘The current protection against severe disease, hospitalisation, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout,’ the CDC said.
 
Professor Collignon does not believe existing data shows conclusively that protection afforded by Australia’s vaccines declines significantly against severe disease – although he says this needs to be watched closely.
 
He is also concerned that booster programs in developed nations will divert vaccines from poorer countries that need them more.

‘The problem with boosters is that is another big supply of vaccine you need, and as WHO [the World Health Organization] points out, you’re giving a third shot in western countries when [some] haven’t even had a first shot,’ Professor Collignon said.
 
‘That’s an equity issue. Also, there is no convincing evidence – if your endpoint is preventing serious disease and death – that you actually need them.’
 
A report on the study in the medical journal Nature concludes that Pfizer is around 92% effective at stopping people from developing a high viral load 14 days after the second dose.
 
Nature reports the study as showing the vaccine’s efficacy declined to 90% after 30 days, 85% after 60 days and 78% after 90 days. AstraZeneca’s efficacy began at 69% a fortnight after the second dose, falling to 61% after 90 days, the journal reports.
 
There was some limited data on the use of the Moderna vaccine – due to be available in Australia from next month – but not enough to draw conclusions on its efficacy after two doses.
 
One of the researchers, Professor Sarah Walker, said the results are encouraging for patients who have been vaccinated with AstraZeneca and Pfizer. But she also said there was not enough data to indicate the exact infectious period for vaccinated people and believes the study reinforces the need for a cautious approach, even after vaccination.
 
‘Both of these vaccines, two doses are still doing really well against Delta,’ Professor Walker told Nature.
 
‘[But] most of our tests are monthly [so] we can’t really say very much at all about how long people are infectious for and particularly whether that’s different with Delta.
 
‘Anyone who thinks that if they get infected having been vaccinated, they can’t transmit — that isn’t likely to be true.’
 
Data included test results from 384,543 people aged 18–64 from 1 December 2020 to 16 May 2021, when the Alpha variant of concern was dominant in the UK, and results from 358,983 individuals from 17 May to 1 August 2021. The second period marked when Delta took over as the main strain in the UK.
 
But while the results seem to provide an accurate recent picture of how Delta has impacted immunity, Professor Collignon believes the true sign of whether hospitalisation rates prove boosters are genuinely required will not emerge until next year.
 
‘There is not really good evidence, even in Israel, where they have done this longest, that you’re really seeing a waning of that type of protection as times goes on,’ he said.
 
‘It may be, but the jury’s out.’
 
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