Is Prime Minister Boris Johnson doing enough to protect us from the new Omicron Covid variant?

The restoration of an obligation to take a PCR test within two days of return to the UK, and to isolate until receipt of a negative result, is the most important of Boris Johnson's announcements today.In theory it will delay the seeding and spread of the new Omicron Covid-19 variant.But the relatively limited prophylactic measures unveiled alongside - compulsory mask wearing in shops and on public transport, the obligation for any close contacts of those infected with Omicron to isolate, whether or not they test positive - will only turn out to be adequate if the new variant hasn't already been seeded here.It is possible that there is more of Omicron in the UK than we know. And if it is, we'll see more of what the PM called his plan B against Covid-19 - vaccine passports for entry to entertainment and hospitality, encouragement to work from home - within days.

Watch the prime minister's press conference in full

One bit of good news is that NHS Test and Trace should be able to establish rapidly the prevalence of Omicron, prior to genome sequencing, because more than one-third of PCR tests are able to distinguish it from the Delta variant (in a slightly crude way, but which allows those tests to be prioritised for sequencing).In other words, we should have more data very soon about the magnitude of the immediate problem we face.There is a lot we don't know about Omicron, but Chief Medical Officer Professor Chris Whitty and Chief Scientific Advisor Sir Patrick Vallance said Omicron is probably more transmissible even than the Delta variant, and that vaccines may be less effective against it.

That said, vaccines we've had, or indeed Covid-19 disease we've already individually contracted, should reduce the risk of severe disease and deaths for those who contract Omicron. Whitty and Vallance said they were hopeful that the more Covid antibodies any of us have in our systems, the greater the prospect that an Omicron infection will be mild.

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That is why the PM is speeding up the booster programme and is why Chris Whitty wants the regulator, the JCVI, to authorise booster shots for everyone aged 18 and over.So our medical defences against Omicron are significantly greater than they were against the initial strain and the early mutations.Even so, you like me probably have a weary sense of deja vu all over again, as it were.I asked the epidemiologist Professor Neil Ferguson to assess the PM's package. He said: "The measures are proportionate as an immediate response" and "I don't doubt that they will be kept under very close review and adjusted - up or down - as the data accumulates".Consider yourselves duly warned (and let’s not jinx Christmas by talking about it).

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