A paper by top scientific advisers on 22 April warned of the need to “maintain low prevalence” of the virus in the long-term.
It said a lengthy period of restrictions may be needed again if it becomes urgently necessary to reduce infection numbers, for example with the emergence of a vaccine-resistant variant.
The paper was published by the government on Monday evening, at the same time as the prime minister announced most mitigations will be scrapped when England’s lockdown ends. That date is currently set for 19 July.
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At his Downing Street press conference on Monday, Johnson acknowledged there may be 50,000 new cases a day by that point. On Tuesday, health secretary Sajid Javid went further and suggested infections could rise to more than 100,000 a day.
However, this is exactly the scenario the paper, which was presented to the Scientific Advisory Group for Emergencies (Sage), warned against.
It says: “There is significant risk in allowing prevalence to rise, even if hospitalisations and deaths are kept low by vaccination.
“If it were necessary to reduce prevalence to low levels again (e.g. variants of concern become more pathogenic for others previously less affected), then restrictive measures would be required for much longer.”
Johnson, for his part, has not ruled out a return to restrictions in the future.
Cases, meanwhile, are already rising exponentially. Some 186,422 new infections were recorded in the week up to Tuesday in the UK, up 49% on the previous seven days.
The paper was published two weeks before the now-dominant Delta variant was escalated to a “variant of concern”.
While vaccines are effective against the 60% more infectious Delta variant in preventing serious illness, scientists have expressed fears that allowing the virus to circulate could lead to further variants which become resistant to jabs.
Prof Susan Michie, a member of the government's Scientific Advisory Group for Emergencies (Sage), said on Monday that “allowing community transmission to surge is like building new ‘variant factories’”.
On the other hand, it has been argued that vaccine-resistant variants will remain a threat anyway, as the virus will continue to spread across less-vaccinated populations around the world.
In an apparent challenge to his critics on Monday, Johnson asked: “If we can't reopen our society in the next few weeks... we must ask ourselves when will we be able to return to normal?"
The paper, meanwhile, also noted:
a sudden opportunity will be presented to the virus in the form of “superspreader” events – such as at nightclubs and religious ceremonies – which have not been possible since March last year
how even countries such as New Zealand with "near-zero" COVID are retaining baseline measures such as requiring masks on public transport. Johnson, who as mentioned above expects 50,000 cases a day by 19 July, will abandon this rule
trying to maintain lower infection rates would have the benefit of reducing the impact of conditions such as "long COVID" and give the NHS more capacity for routine care. Johnson openly acknowledged COVID infections will increase when lockdown ends
COVID transmission will be “highly likely” to increase in the autumn and winter as people spend more time indoors and close windows due to the colder weather
However, England’s chief medical officer Prof Chris Whitty, the UK's most prominent COVID scientist, said this last factor is a reason why he has “quite a strong view” lifting lockdown in the summer has advantages over autumn.
He said autumn will be “when schools are going back and when we’re heading into the winter period when the NHS tends to be under greatest pressure for many other reasons”.
On Tuesday, the government announced a further easing of the rules, with self-isolation requirements for people who have received both vaccines to be dropped from 16 August.
It means fully-vaccinated people who have come into contact with a COVID carrier will not have to isolate for 10 days. The same will apply for under-18s, an age group which is not being vaccinated.
Health secretary Sajid Javid told MPs that the new measures provide a "proportionate" way of managing the pandemic.
The “bubble” system in schools will also be scrapped following concerns about large groups of pupils being sent home if a case is detected.
Transport secretary Grant Shapps, meanwhile, will update MPs later this week on how the government will remove the need for fully-vaccinated arrivals to isolate when they return from an "amber list" country.
Watch: Double-jabbed contacts will not have to isolate from 16 August