Why is the government so scared of the Omicron variant? So far, most of the evidence we have for transmissibility and virulence of Omicron is based on very limited data from South Africa, but the UK Health Security Agency (UKHSA) has now published its own preliminary study of the variant — the results of which will presumably have been available to ministers and scientific advisers prior to Wednesday’s decision to enact ‘Plan B’. They appear to show a variant which is more transmissible, more likely to evade vaccines and more likely to reinfect people who have previously had Covid. But there is a very big caveat: they are based on tiny numbers of Omicon cases. As for the severity of Omicron, which evidence from South Africa suggests may be lower than that for previous variants, with far fewer patients requiring oxygen, the UKHSA offers no evidence yet, saying this is still several weeks away.
The UKHSA data is based on just 581 cases of Omicron. It suggests, at face value, that Omicron is more likely to spread to another household member compared with Delta. However, this needs to looked at in the context of the actual figures: the study found that in 121 cases of Omicron, 23 (19 per cent) were linked to a second case in the same household. The study compared this with 72,761 cases of Delta, 6058 (8.3 per cent) of which were linked to a second case in the household. The UKHSA says it has adjusted this data to take into account the size and composition of households to come up with odds ratio of 3.2, meaning you are over three times as likely to catch Omicron from a household member as you are Delta. However, this is based on a very small number of Omicron cases, and there is a possibility that they are concentrated in households with a particular characteristic which has not been adjusted for.
The UKHSA has analysed five preliminary studies (two from Britain and three from abroad) to come up with the suggestion that there is a 20 to 40 fold reduction in immune response when an individual who has been double-vaccinated with two doses of Pfizer is exposed to the omicron variant, compared with someone exposed to the Delta variant. These are a laboratory studies on blood samples rather than real-life studies. The reduction in immune response was even greater among those who had received two doses of AstraZeneca — with immune response ‘below the limit of quantification’ in some cases.
The UKHSA also offers some real world data on the efficacy of vaccines against the Omicron variant — although this, too, is based on a very small number of cases (581) and, as the UKHSA warns, need to be treated with caution for that reason — there is a very wide margin of error. It suggests that the chances of going down with symptomatic disease from Omicron do decrease sharply after a booster dose. Two weeks after receiving a Pfizer booster jab, it claims, the chances of symptomatic infection fall by around 75 per cent (in the case of people who received first and second doses of Pfizer) and by around 70 per cent (in the case of people who received first and second doses of AstraZeneca. Last year’s clinical trials of the Pfizer vaccine put the efficacy against symptomatic disease (from the original variant of the SARS-CoV-2 virus) at around 90 to 95 percent.
The UKHSA figures might explain why the government has tried to act quickly with new restrictions: they attempt to quantify the bad news, but they do not try to quantify — nor even analyse in any way — the potential good news that Omicron might cause a milder disease than previous variants. If the general thrust of the South Africa data is reflected in future studies by the UKHSA and others in Britain, we may see a sharp change in attitude.