Yesterday the UK Health Security Agency (UKHSA) announced it had identified another 26 Omicron cases, and the total number of cases in the UK had reached 160.

The rate of increase from zero in little over a week seems significant. But the one thing we know is these official figures are a significant underestimate of how many cases are actually in the UK.

Here’s why.

First, you will recall that last week I reported UKHSA’s statement to me that only 50 per cent of pillar 2 or community testing can assess Covid-19 samples for s-gene target failure – which means only half of the labs pick up that initial clue (not proof) that any given positive result is Omicron rather than Delta.

So for the other half of labs, there is no way yet of screening positive results to determine whether they should be sent on for the gene sequencing that establishes whether any particular infection is indeed Omicron, or a different strain (UKHSA told me they were taking steps to address this shortcoming).

Second, only between 15 per cent and 20 per cent of all Covid-19 positive tests are sent for gene sequencing in any case.

Third, gene sequencing can takes days. And when a virus is doubling as quickly as this one is – every three days, according to data from South Africa – then an average three-day wait for sequencing results means UKHSA is reporting on cases that happened three days ago and ‘meanwhile the epidemic has doubled’ (in the words of an epidemiologist).

Fourth, according to one government adviser: ‘when we compare estimated infections as measured by the ONS survey and reported cases we find that we generally report about a third of infections. So, the true number of Omicron infections would be inflated by this number as well’.

All of this means that, as I said, Omicron is already with us in much greater size than we know. There will already be significant transmission within communities.

And in the absence of knowledge of how dangerous or severe Omicron is, the precautionary principle must apply – that both government and individuals have a powerful responsibility to take the necessary steps to protect ourselves and each other.