Omicron: Why it doesn't really matter if new Covid variant causes milder disease

The Scottish Government evidence paper projects a 'central' scenario of around 5000 Omicron cases a day by December 20, but it could exceed 15,000 in a worst-case scenario

Yet hospital data from South Africa published last weekend seemed to indicate that the Omicron wave was causing milder disease compared to previous Delta and Beta waves.

Far fewer Covid patients were on oxygen treatment; just two per cent were requiring ventilation compared to an average of 11% during Delta; and three weeks into the Omicron surge, the number of Covid patients in intensive care was a quarter of what it had been at the same time with Delta.

Scientists plotting the Omicron curve have noted that cases are rising much faster than previous waves, yet hospitalisations are rising more slowly.

But this is not as clear cut as it seems.

If a variant is much faster-spreading then the cumulative number of people infected by the time it reaches 7000 cases a day will be smaller than with a less transmissible predecessor.

This alone would skew proportions of ICU admissions and deaths at this stage.

That is, the proportion of cases we’ve seen thus far in the super-fast Omicron wave is smaller than the proportion of cases we’d seen at this point in the slower Delta wave. This factor alone could cause an unusually low proportion of severe/ICU at first, says @dr_kkjetelina. pic.twitter.com/d8HEfXOTFn

— Gabrielle Babbington (@glbabbington) December 8, 2021

Secondly, we know that vaccination reduces the infection-to-hospitalisation ratio.

A much smaller proportion of infected people in Scotland were ending up in hospital during the Delta wave not because it was milder, but because a majority of adults were vaccinated - and thus more protected against severe disease.

READ MORE: Scots told to 'think carefully' about socialising as 'tsunami' of Omicron cases looms

There is also evidence that Omicron has a greater capacity to re-infect. Although only a quarter of South Africa's population is fully vaccinated, many more have recovered from prior infections which mean that they are also more likely to experience mild disease.

A sketch to explain how a new variant may appear milder even with no change in underlying virulence. This can occur because, when calculating the fraction of cases that are severe, the denominator now includes many re-infections that had previously been averted. A thread. 1/8 pic.twitter.com/XxrYHnb6XY

— Natalie E. Dean, PhD (@nataliexdean) December 9, 2021

So, even if Omicron is just as virulent as Delta, if its mutations make it more capable of causing repeat infections in the recovered or breakthrough infections in the vaccinated, then the proportion of total cases that are mild will be greater.

As John Burn-Murdoch, a Covid statistician and fellow at the London School of Economics, put it: "What we’re seeing with Omicron may be best understood not as: 'this variant makes you less sick', but as 'loads of people who would not have been infected at all with other variants, are now getting mild infections [which pushes the severe shares of cases down]".

"Lots of people are still getting very sick in South Africa, it’s just that they’re being drowned out in the stats by the deluge of mild cases among those with prior infection or vaccination."

This may explain the trends in South Africa, along with demographics (its population is comparatively young - just 6% are over 65) and the fact that cases are still concentrated in under-50s who are less likely to get sick.

But to go back to the point that milder illness doesn't really matter: this seems counterintuitive, but it is a question of mathematics.

A variant which is more transmissible will actually cause more deaths than one which is equivalently more lethal, simply because of its capacity to reach a huge number of people - a small percentage of whom will inevitably die.

This was illustrated in a hypothetical scenario drawn up by Adam Kucharski, a London-based epidemiologist and mathematician, which demonstrated that starting from a base of 10,000 initial infections a virus that is 50% faster would cause 978 deaths after a month compared to just 193 from a variant that is 50% more lethal.

READ MORE: Half of Omicron cases in Scots aged 20 to 39

The final problem with Omicron is its capacity to erode vaccine protection against infection.

According to preliminary data from the UK Health Security Agency, Omicron has "substantially reduced" boosters' ability to stop mild disease (that is, symptomatic infections) from around 95% with Delta to 75%.

That means there would be five times as many infections among boosted people in an Omicron wave than there would have been with Delta (because 25% of people will now be susceptible, instead of 5%).

Protection against severe illness is expected to remain high but, once again, it is a matter of numbers: if the virus is more transmissible, and able to infect many more people who would previously have been protected, then a "tsunami" of cases will very quickly wash up in an NHS that is already at capacity.