2 Variants of concern

Thousands of different genetic variants of SARS-CoV2 have been sequenced. A number of these are identified by the World Health Organisation (WHO) as variants of interest (VOI), perhaps because they have spread locally or have mutations that are characteristic of pandemic strains. If a strain spreads more widely it may be designated as a variant of concern (VOC).

A VOC strain does not necessarily have all of these characteristics. For example the omicron strain has greatly increased transmissibility over previous strains, but produces less severe disease than the delta strain. Also none of the VOCs to date have been able to completely evade the PCR detection tests or the lateral flow tests.

One common feature of later VOCs was that they could partly evade antibodies produced by infection with earlier strains or the initial vaccine formulations which were based on the sequence of the Wuhan strain spike protein. In addition, mutations that allowed a strain to replicate more quickly or spread more effectively, provided a selective advantage and were seen in later VOCs. The effect of increased transmissibility is seen in table 8.2, which gives estimated R₀ values for different VOCs. Notice that there is a range of estimated values due to the difficulties of estimating R₀ in different populations, discussed earlier.

Remember that R₀ is the rate of spread in a totally susceptible population, so these values represent the basic transmissibility of the different strains. It is noticeable how the alpha delta and omicron strains which caused major waves of infection in the UK all have increasingly high R₀ values. In comparison the gamma and epsilon variants, which did not spread in the UK, have R₀ values similar to the original Wuhan strain.