The Omicron variant advances at an incredible rate
Even if infections prove mild, that speed will have grave consequences
EXPONENTIAL GROWTH is a dizzying thing. In the week to December 8th Britain saw 536 new cases of covid-19 ascribed to the Omicron variant, less than 0.5% of the number caused by the dominant Delta variant. But the week before there had been only 32 cases of Omicron—and by December 14th the case number was over 10,000. Omicron looks set to become the country’s dominant strain in terms of cases before advent calendars run out of windows.
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Cases lag behind infections. On December 13th Britain’s health minister, Sajid Javid, said that there were estimated to have been 200,000 infections in the country that day, most of them Omicron. Three more doublings and the total number of infections a day will be more than 1m.
It is not just in Britain that Omicron is outstripping Delta. It has already displaced it in South Africa, where it first came under scrutiny and where its growth, though possibly now slowing a little, has been spectacular. Studies in various European countries show similar growth, though with a later start. The same is true in America. The variant has now been detected in almost all countries, including China. The numbers involved are often small. But with exponential growth small is not much comfort: it doesn’t last.
Omicron seems to have two attributes that enable such rapid spread. Some subset of its many mutations seems to make it more transmissible. Contact-tracing studies in Britain have found that the risk of a given Omicron infection spreading is two to three times that for a Delta infection.
And because it is better at infecting people who have previously been vaccinated, or infected, it has a larger pool of people to infect. On December 14th Discovery Health, South Africa’s largest health insurer, produced the initial results of work on Omicron undertaken with the South African Medical Research Council. They found that, whereas two doses of the Pfizer-BioNTech mRNA vaccine offered 80% protection against infection before the Omicron wave, against Omicron the protection dropped to 33%. Its effectiveness in reducing the risk of severe disease is also lower: two doses of the Pfizer vaccine reduced the risk by 70% (down from 93% for Delta).
That still makes being vaccinated a very good thing. Most of the people who have been hospitalised with Omicron in South Africa, and 84% of those in intensive care, are unvaccinated. And it does not look as if the sort of vaccine someone was given matters all that much. The other vaccine in use in South Africa, made by J&J, an American company, relies on a modified adenovirus, rather than mRNA, to get its message into the body, as does the more widely used AstraZeneca vaccine. It, too, seems to protect against severe disease.
These real-world findings support the hope that two doses of all the existing vaccines will continue to offer significant protection against severe disease, even if they are not so good at blocking infection. That makes sense. Evolution has shaped the immune system to reduce the risk of death. Stopping a virus which the body has seen before from infecting it again is a helpful step in that direction, and one of the primary purposes of the antibody response. But it is not the only step; the defence is deep and layered. Incoming Omicron may be good at evading the antibodies produced by vaccines and previous infections—it is 3-8 times more likely to infect someone who has previously been infected than other variants. But further foes await it.
After infection has started, cell-based immunity joins in the fight, seeking out and destroying the cells which the virus has suborned. This response is greatly strengthened by prior vaccination. And it is less easily fooled by a few changes to the proteins on the virus particles’ surface. That is why vaccines can still protect against disease even if the antibodies they provoke no longer recognise the pathogen as well as they did originally, or if they have waned over time.
That said, a better antibody response would be nice to have; slowing the rate of infections would slow the spread of disease and give health systems breathing space. This is where vaccine booster shots come in. Boosters improve all forms of immunity: one of their effects is to raise antibody levels, at least for a while. This increased quantity can go some way to making up for the reduced quality of their response, lowering the risk of infection. Boosters may improve the quality of the antibodies, too; the more the immune system sees a virus, the better attuned to it some antibodies become.
A reasonable expectation that vaccines offer protection against serious disease, especially after a third jab, is one piece of good news. Another may be that Omicron infection leads to less severe disease all round. There is some evidence of this from Gauteng, the South African province where the variant has run rife. Data from Discovery Health suggest adults with Omicron have a 29% lower hospital admission risk relative to that seen in the country’s first wave of covid-19 in mid-2020. The proportion of those hospitalised who end up in intensive care is much lower than in previous waves, too, and fewer of those on general wards need supplemental oxygen. Angelique Coetzee of the South African Medical Association, who was one of the first to raise the alarm about Omicron, has consistently argued that it is a milder variant.
A paper recently submitted for peer review by Michael Chan and colleagues at Hong Kong University suggests one reason why this might be. They found that in the first few days of infection Omicron reproduced 70 times more readily than Delta in the airways leading to the lungs. But in the lungs themselves it reproduced ten times less well than earlier variants.
The details of how viruses cause disease depend on a lot more than simple reproductive rates. But this finding might go some way to explaining a lower incidence of severe disease; it is infection in the lungs that does most damage. And greater replication higher up the respiratory tract might improve transmissibility. Being very good at getting into, and reproducing in, the lining of the airways could make it easier for the virus to set up shop in someone exposed to it. What is more, a lot of activity in the airways might also mean more particles get back out into the air. Indications that the symptoms of Omicron infection are more like those of the common cold might fit with this interpretation.
But neither that laboratory work nor the data from South Africa amount to a strong case that Omicron will be a lot less dangerous than earlier strains everywhere. The South African data are preliminary; so far they cover only the first three weeks after infection. Typically, new waves of covid variants start in younger groups and work their way into older, more vulnerable populations over time. And the rates at which infection leads to severe disease and death can differ between countries and populations. Factors at play beyond the youth of the South Africans might include the fact that most vaccinations in the country are relatively recent and the fact that a fair number have been previously infected. There may also be salient factors stemming from genetic variation or prior health histories. Things could look quite different in older populations elsewhere in the world which have seen fewer infections.
Da capo, molto vivace
The degree to which the variant can infect the previously infected may also make things look rosier than they really are. Natalie Dean, a biostatistician at Emory University, points out that Omicron’s success at reinfecting people may give the impression a smaller fraction gets severely ill just by inflating the denominator. It could thus seem more benign even if, among those contracting covid for the first time, it were just as dangerous as Delta (see chart).
As the debate about the comparative severity of the infection goes on, public-health officials are stressing that what matters to the individual and to the health system are not all that well aligned. For an individual, a less deadly variant is preferable to a more deadly one, regardless of how transmissible it may be. For a health system, the number of cases at any given time is a critical concern, which makes the rate of transmission crucially important. There is a level beyond which the system cannot cope with the number of hospitalisations. A fast-spreading virus can reach that level even if it produces a lower proportion of severe cases simply because the total number of cases at any given time is so high.
To provide a sense of this, researchers at the London School of Hygiene and Tropical Medicine have compared models for the spread of Omicron with the situation in England during the worst previous peak, in early 2021. Of their various scenarios, the one which currently looks most plausible makes Omicron pretty good at infecting people who have been vaccinated or infected but also treats boosters as being quite good at stopping it. That would produce a peak in hospital admissions in late January well over the 3,800 a day seen in 2021. It would lead to 23m-30m infections between now and May 2022, and 37,000-53,000 deaths. Models from the same team have, in the past, proved overly gloomy, but they believe they understand why and have made appropriate adjustments.
The prospect of hospitalisation rates that high saw England go into two national lockdowns, one in November 2020 and one in January 2021. This time the government has recommended working from home and reintroduced some infection-control measures, such as mask-wearing on public transport. It may introduce more. Its greatest stress, though, is on a hell-for-leather dash to provide booster shots to all adults by January 1st.
In a report published on December 15th the European Centre for Disease Prevention and Control warned that a range of enhanced precautions were now necessary, including reduced contact in social and work settings, fewer large gatherings, more mask-wearing and more testing. Reducing travel and mixing between households and generations over the holidays may also be on the cards. Some countries are sure to see tougher measures soon; some individuals are already taking them. But if the perception that Omicron is not so dangerous—whether well founded or not—becomes widespread, people may see little reason to adhere to stricter rules.
Countries where Omicron rates are still very low have a little more time to prepare, to learn from those further along the curve, and to estimate what is necessary to flatten and lower the peak. But the growth rates seen so far strongly suggest that time is best measured in days, maybe weeks. Exponential growth is a dizzying thing. ■
