This broadly makes sense. Negative LFTs on days six and seven should indicate if a person is past the peak of their infection. The change will prevent people from isolating when they are no longer infectious at a time when omicron is pushing cases to record levels. The government is even considering whether it might be possible to shorten the isolation period further still.
However, shortening isolation is not without contention. LFTs are not 100% accurate at identifying infectious people – a small number will test negative at day seven and still pose a risk. Note too that this guidance specifically references LFTs; a PCR test would still be positive so soon after catching the virus.
A further change, in force from January 11 in England, is that people who test positive on an LFT should continue to isolate immediately but are no longer required to take a confirmatory PCR test.
The motive here seems to be to prioritise getting people who are likely to be infectious to isolate, but not worry about confirming that they definitely have Covid or which variant of the virus they have (PCR tests can also be used to distinguish between variants). Again, with infection levels so high, this seems like a sensible move that continues to contain transmission but also, maybe this time, saves testing resources.
Finally, fully vaccinated travellers arriving in the UK used to have to take a PCR test two days before travelling and another within two days of arrival. Now neither is required – people arriving just need to isolate for two days on arrival and take an LFT on day two (and if the LFT is positive, continue to isolate and take a PCR test).
Again, this makes sense – it will identify infectious people and get them to isolate, but it won’t needlessly limit those who have recently got over Covid but would still test positive using a PCR. The requirement of a follow-up PCR test if the LFT is positive potentially allows for authorities to see what variant of the virus it might be (as there’s always the possibility that new variants will arrive from abroad).
But for travellers who are not fully vaccinated, the rules are different. They still need to take the more sensitive PCR tests prior to travel and on days two and eight after arriving. This decision is slightly harder to understand, because while vaccination protects people from getting seriously ill, it isn’t highly effective at stopping people catching the virus, particularly with the omicron variant.
The logic of more stringent testing of the unvaccinated when omicron is so broadly infectious is therefore unclear. It may be that this decision is more a tactic to try to convince more people to get vaccinated, rather than being a decision based on an entirely scientific rationale.