Dealing with uncertainty about the Omicron variant

The race is on to understand the new variant identified by scientists in South Africa and Botswana, dubbed Omicron (the next Greek letter was “nu”, but this could have been mistaken for “new”). Fears include greater spread, worse disease or reduced effectiveness of treatments and vaccines.

Increased transmission can arise from two factors. First, there is an intrinsic advantage, with a heightened “basic reproduction number” R0; in a susceptible population, that is the average number of people each case infects, although after 20 months of pandemic this has become a notional concept. It was around 3 for the original wild-type virus, compared to around 6 for Delta and possibly rather more for Omicron.

Second, a variant can show better evasion of existing immunity, whether from vaccination or past infection. Only one in four South Africans has had two doses of Covid-19 vaccines, but the country has already endured three major waves and an estimated 270,000 excess deaths since May 2020. Despite that wall of existing immunity, both confirmed cases and test positivity have surged. Such increased transmission could arise from different combinations of these two factors, but increased ability to evade immunity is concerning.

There have been some claims that Omicron infections mean milder cases, but lags between infection and attending hospital make this difficult to assess rapidly and hospital admissions with positive tests are rising in Gauteng province. Like compounding interest, increased transmissibility means more cases generation after generation and even a reduced share going to hospital may still result in more admissions. And an extraordinarily rapid analysis indicates the risk of reinfection is substantially higher than in its first wave.

Related: Sage scientists seek balanced UK response to Omicron concerns

With these possibilities, it is reasonable to be cautious while we wait for basic uncertainties to be resolved. Like Alpha, which spoiled last Christmas, the progress of the variant can be roughly monitored from its specific pattern of PCR test results and UKHSA reports that an army of scientists is already planning 20 studies. Soon this article will be ancient history.

Judging the swift domination of the variant in South African sequences, the age of Omicron has begun.

• David Spiegelhalter is chair of the Winton Centre for Risk and Evidence Communication at Cambridge. Anthony Masters is statistical ambassador for the Royal Statistical Society