The reasons why the omicron strain of the coronavirus appears to be milder

Omicron is not harmless. In fact, the latest studies suggest that the coronavirus variant that is sweeping across the world is not radically less severe in itself; it is simply that omicron is infecting a population with different characteristics. Now the delta variant, which was identified a year ago in India, and omicron, detected a month ago in South Africa, coexist and scientists are trying to compare the risk of hospitalization among similar demographics, half infected with omicron and the other half with delta.

When analyzed, it appears that the risk of the new variant is lower, but a study from Imperial College London published just before Christmas reveals that up to 40% of omicron cases could be people who have already had Covid-19, compared to 8% with delta. Both variants are highly contagious among the unvaccinated, but omicron is also capable of infecting a significantly larger number of people who already have good defenses, making it seem less serious.

The question is whether the new variant is in fact milder than previous ones. US biologist Marm Kilpatrick from the University of California puts it this way: “The short answer is that we don’t know yet.” The Imperial College report shows that the likelihood of ending up in hospital with omicron is up to 45% lower than with delta. However, without taking into account the effect of the higher number of mild cases resulting from reinfections, the authors of the report estimate that the probability of ending up in hospital with omicron could be only 2 to 35% lower than with delta. “This small difference in severity would be far outweighed by the large number of cases caused by omicron, which will result in many hospitalizations. If it really is milder, it’s not that much milder, so it’s not a reason to be complacent about the increase in cases of omicron,” Kilpatrick adds.

The problem is the sheer volume of the current wave. If the probability of ending up in hospital with omicron were even 50% lower than with delta, with cases doubling every two days, there would be the same number of Covid admissions within the space of two days, according to epidemiologist Deepti Gurdasani, from Queen Mary University of London. And cases of omicron have indeed been doubling every two days in many countries. The Institute for Health Metrics and Evaluation at the University of Washington estimated on December 22 that three billion people will be infected with the new variant in the next three months. If these estimates are confirmed, there will be as many cases in the next quarter as there have been over two years.

If it is in fact milder, it’s not that much milder, so it’s not a reason to be complacent

Marm Kilpatrick, US biologist

Aside from the lack of certainty regarding the actual intrinsic severity of omicron, the good news is that its effects in the current climate are less severe, thanks to a combination of vaccination, natural defenses from previous infections, and the possible milder characteristics of the virus. The University of Washington models estimate that 90% of omicron cases will be asymptomatic, compared to 40% of cases infected with earlier variants.

According to the Imperial College analysis, people with at least two doses of the Pfizer-BioNTech, Moderna or AstraZeneca vaccines remain “substantially protected” against severe Covid-19, although omicron has a greater ability than delta to cause mild infections in those who have been jabbed. The study shows that severe symptoms are 80% less likely in those with two doses of Pfizer than for those unvaccinated and infected with the delta variant. With two doses from AstraZeneca, this figure falls to 65% lower. A third booster jab also increases protection, though it tends to decrease over time.

South Africa’s National Institute for Infectious Diseases estimated on December 21 that the risk of hospitalization from omicron is 80% lower than with delta, but its study compares the current wave with that of five months ago, when the percentage of vaccinated individuals was lower and there was also less natural immunity from previous infections. The South African study does not compare the severity of current cases of omicron and delta, as the Imperial College analysis does.

A preliminary analysis of data from Scotland, meanwhile, concurs with the Imperial College study. Omicron cases have 60% less risk of ending up in hospital than delta cases as the new variant has a greater ability to infect people who already have natural defenses. The Scottish study, published by the University of Edinburgh on December 22, suggests that the third dose of the vaccine reduces the risk of having a symptomatic infection by 57% compared to having just two doses administered more than six months ago. The authors also estimate that the ability of omicron to reinfect people who have already had Covid-19 is 10 times that of delta.

There is not enough data to know whether omicron is intrinsically less serious than delta

Frode Forland, Norwegian doctor

One of the most rigorous studies of an omicron outbreak focused on a party on November 26 in Oslo, Norway. At least 81 of the 117 party-goers became infected and there was only one asymptomatic case. The others, who averaged 38 years of age and were fully vaccinated, mostly suffered from coughs, fatigue, runny noses, sore throats and fever. Norwegian doctor Frode Forland explains that there is no record of hospital admissions, but stresses that it is too early to draw conclusions. “There is not enough data to know whether omicron is intrinsically less serious than delta,” says Forland who works at the Norwegian Institute of Public Health.

Forland says he is “very concerned” about the overwhelming transmissibility of the new variant. “So many people infected in such a short time will cause a very high peak and we will have trouble treating so many people, even if omicron is in fact milder,” he says. Epidemiologist William Hanage, from Harvard University, US, offered a graphic explanation of how the numbers work to the British newspaper, The Guardian: “The difference between a caress and a slap is largely in the pace with which it is given.”

Two epidemiologists from Madrid’s La Princesa hospital, Joan Soriano and Julio Ancochea, have, together with two Italian colleagues, calculated that the peak of this wave will be reached on January 15 in Europe, with 400,000 cases per day. “The debate on whether omicron is more or less serious is pointless, because the virus is still very bad. It can send you to intensive care and kill you,” Soriano says. “What’s more, primary care and hospital outpatient clinics were already saturated, but now they are overwhelmed.” In London, 7% of doctors are on sick leave due to Covid-19, either because they have it or because they have been in close contact with someone who does.

Soriano is also concerned about so-called “long Covid,” when symptoms can linger for months and patients are dogged either by respiratory problems, fatigue or a mental fog that affects concentration. “One in six symptomatic infections ends up in long Covid,” says Soriano, citing data from previous variants, because it’s still too soon to know the long-term effects of omicron. “People play the Christmas lottery thinking they are going to win, even though the probability of winning the jackpot is one in 100,000. However, they assume they won’t get long Covid, which is one in six. Humans assess risk very poorly,” he adds.

Mathematician Sarah Rasmussen, a pandemic analyst at the University of Cambridge in the UK, also believes it is too early to tell whether the new variant is in fact less severe, all else being equal. “There seem to be slight indications that omicron might be inherently milder, but in my view there is not enough evidence,” she says. “To answer this question properly, we will have to look at future omicron and delta data in countries where there are substantial numbers of older people who are unvaccinated and have not had previous infections. For example, it is possible that the US will eventually generate such data.”