[Lewis Thomson (OP)]

Sheila would like some help understanding this situation.

"I have seen reports from Pfizer that there is a 25 fold reduction in protection from two dozes of Pfizer against the Omicron variant. I have also seen reports from South Africa stating that there is a 20-40 fold reduction. Can you please explain what this means in terms of protection in percentages after two doses of Pfizer?"

Can you help them? Leave your answers in the comments below...

[alancalverd]

Depends on the exact statement.

If it was  25-fold reduction after only two shots, then it should be interpreted as "a booster will increase protection by a factor of 25".

Alternatively it could mean that omicron is 25 times more infective than previous variants despite standard 2-shot vaccination.

Neither statistic strikes me as particularly plausible.

[evan_au]

The Pfizer COVID vaccine was developed against the original Wuhan strain of COVID-19, and develops antibodies against the Spike protein; they claimed a protection of 90% against infection in clinical trials. It was also fairly effective against the Delta variant.

However, the Omicron variant has a large number of mutations (including many on the Spike protein). That means that antibodies prompted by the Pfizer vaccine are unlikely to neutralise the spike protein of the Omicron virus (ie unlikely to prevent infection with Omicron).

In Australia, around 90% of the population was double vaccinated (mostly using Astra-Zeneca or Pfizer). However, the peak infection rate with Omicron was about 50 times higher than the peak infection rate with Delta (almost certainly an underestimate, as the testing services were totally overloaded with Omicron, and just threw the out-of-date unprocessed nasal swabs in the bin).
https://ourworldindata.org/coronavirus/country/australia

These researchers "report a 17–22-fold reduction in neutralisation titres in laboratory tests" against Omicron:
https://www1.racgp.org.au/newsgp/clinical/omicron-a-significant-challenge-to-two-dose-vaccin