COVID Vaccine Effectiveness 'Waning Over Time'

Peter Russell

August 25, 2021

Protection provided by two doses of the Pfizer-BioNTech and the AstraZeneca/Oxford COVID vaccines wanes within 6 months, according to new research.

An analysis by the ZOE COVID study suggested that a "worst case scenario" could see protection reduced to below 50% for healthcare workers and the elderly, who were among the first to be vaccinated.

Initial protection against SARS-CoV-2 for the Pfizer vaccine a month after a second dose was 88%. After 5 to 6 months that decreased to 74%, suggesting that protection fell by 14% in 4 months.

With the AstraZeneca vaccine, there was protection against infection of 77% a month after the second dose. After 4 to 5 months, that fell to 67%, suggesting protection was reduced by 10% over 3 months.

Infection risk reduction since the end of May in AstraZeneca and Pfizer/ZOE

The research, presented as a press release, was based on more than 1.2 million test results and information from participants of the ZOE COVID app who log in to contribute to the 'real time' research.

Initial results from a recent preprint study funded by Pfizer suggested that efficacy of its vaccine peaked at 96.2% after a second dose, reducing to 83.7% after 4 months.

Delta Variant

The ZOE analysis suggested that a lower level of efficacy should be expected in a real-world analysis compared with clinical trials, which were conducted before the more transmissible Delta variant became dominant.

The researchers said that although protection appeared to decrease steadily, a person's actual risk could vary due to individual variation in antibody duration.

Prof Tim Spector/ZOE

Prof Tim Spector, lead scientist on the ZOE COVID Study, said: "In my opinion, a reasonable worst-case scenario could see protection below 50% for the elderly and healthcare workers by winter. 

"If [there are] high levels of infection in the UK, driven by loosened social restrictions and a highly transmissible variant, this scenario could mean increased hospitalisations and deaths."

He called for a decision to be made on whether to rollout booster vaccines, as well as a strategy on whether to vaccinate children.

A 'Finger in the Air Prediction'

Commenting on the findings for the Science Media Centre, Paul Hunter, professor in medicine at the University of East Anglia, said it was not clear "whether this decline in effectiveness is based on symptomatic illness or whether they have identified declining effectiveness against severe disease".

He cautioned that there was currently "no strong evidence that immunity to severe disease wanes substantially over the same time scale or that vaccines are less effective at reducing the risk of severe disease from the Delta as opposed to the Alpha variants".

Dr Peter English, a retired consultant in communicable disease control, agreed that there was "a world of difference between efficacy against, on the one hand, any infection and on the other hand, illness severe enough to require hospitalisation, critical care, or to cause death".

Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, said: "The claim that immunity levels will hit around 50% by Christmas is not based on any robust analysis of data, and seems more like a finger in the air prediction."

However, the data was "a reminder that we cannot rely on vaccines alone to prevent the spread of COVID", he added.

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