In the simplest of terms, when its R0 drops below 1.0, a virus (as an outbreak) begins to sputter and dies out. Above 1.0, and an outbreak can have `legs’.
Calculating the R0 is notoriously difficult, however since much hinges upon the existence and subtle differences between viral strains, the accuracy of surveillance and reporting, `seasonality’ ,individual host responses to the virus (i.e. number of `super spreaders’), and how diligent the community is in following social distancing, mask wearing, and hand hygiene recommendations.
But as I mentioned yesterday (see UK PHE Technical Briefing, Revised Risk Assessment, & Household Transmission Study On COVID Variant B.1.617.2 (Delta)), cases have been rising as the B.1.617.2 (Delta) Variant has gripped the nation.
Whether the decision has been made, or this is a `trial balloon' to test the pandemic weary public's acceptance of another delay, remains to be seen.
But late this week Professor Neil Ferguson, an Epidemiologist at Imperial College London warned:
by Kate Wighton, Stephen Johns 10 June 2021
Professor Neil Ferguson says there is a risk of a substantial third wave of COVID-19 infections in the UK, driven by the Delta variant.
"There is a risk of a substantial third wave – we cannot be definitive about the scale of that, it could be substantially lower than the second wave, or it could be of the same order of magnitude."Professor Neil Ferguson Director of MRC-GIDA and the Jameel Institute
The Delta variant, also known as B1.617.2, is thought to be between 30-100% more transmissible than the Alpha variant, also known as B.1.1.7.
Professor Ferguson was speaking at a Science Media Centre briefing with Imperial’s Professor Wendy Barclay and Dr Jeffrey Barrett from the Wellcome Sanger Institute.
Professor Ferguson said: “There is a risk of a substantial third wave – we cannot be definitive about the scale of that, it could be substantially lower than the second wave, or it could be of the same order of magnitude.”
The size of the wave will depend on how effective vaccines are at protecting people against admission to hospital and death, as well as a few other issues, he added.
Prof Ferguson said it was currently difficult to resolve how cases “will translate into hospitalisations, but it’s well within the possibility that we could see a third wave at least comparable in terms of hospitalisations, maybe not as severe, as the second wave.
“Almost certainly I think deaths probably will be lower – the vaccines are having a highly protective effect, cases in hospital now are milder – but it still could be quite worrying.”
Growth of Delta variant
Professor Ferguson said that the Delta variant is growing in nearly all local authority areas, with an estimate for the reproductive value (R number) of the variant put at 1.5-1.6.
The current doubling time for the variant is just under a week, which is 'comparable with what we saw before Christmas', he added.
Professor Ferguson said: "The key issue is how long will it keep on doubling, because we’re starting at a very low level and we have a lot of immunity in the population from vaccination and from people being infected in the past."
Delaying the roadmap
Vaccine efficacy for the second dose against the Delta variant, which was first identified in India, is higher than after one dose.
The next stage of the roadmap out of of lockdown for England will see all restrictions lifted on 21 June.
Professor Ferguson said a delay to the roadmap would allow more people to be vaccinated, and would also have an effect on transmission.
Virologist Professor Barclay, from the Department of Infectious Disease, and lead of the G2P-UK National Virology Consortium, added: "Any delay, from a purely scientific basis, will help because it will allow more time for people to get the second dose.
"Just having it isn't enough, you need about seven days for the vaccine to really boost the immune response back up to the levels that we'd like it to be."
Professor Ferguson said that the upcoming school holidays would reduce contacts in the population slightly and that will counterbalance any increase caused by going to step four of the UK road map'.
The dilemma facing the UK - where the Delta variant has gone from undetectable to massively dominant (the most recent data show 74% of sequenced cases and 96% of sequenced and genotyped cases are Delta) in less than 3 months - is one that other countries (including the United States) are likely to face in the months ahead as the Delta variant expands globally.
While there are some concerns over reduced efficiency of the vaccine against the Delta variant, the data so far suggests that (fully vaccinated) individuals are likely to derive significant protection, both in reducing the risk of infection and the severity of illness.
The Delta variant is already here in the United States - albeit at low levels - but it is growing faster than any of the other variants. Barring a miracle, it should become dominant here in the next couple of months, just as it has in the UK.
How much of an impact that will have next fall will depend largely on vaccine uptake over the summer.
