Omicron is the fastest-spreading virus known to humankind. Barely a month after its detection in southern Africa, the new variant of the coronavirus was already dominant in countries around the world, with more cases than ever before. “It’s an incredibly rapid spread,” said physician Roby Bhattacharyya, an infectious disease expert at Massachusetts General Hospital (USA).
To demonstrate omicron’s infectious power, Bhattacharyya has done a back-of-the-envelope calculation to imagine what a race between omicron and measles, another of the world’s most-contagious viruses, would look like. One person with measles infects 15 others on average if none are vaccinated, compared to the six people infected by omicron. But the key lies in the so-called “generation time”: i.e. the number of days that elapse between when the first person is infectious and when those they infect also become infectious. With measles, that takes about 12 days. In the case of omicron, this only takes four to five days. “One case of measles would cause 15 cases within 12 days. One case of omicron would give rise to another six at four days, 36 cases at eight days and 216 after 12 days,” explained Bhattacharyya.
In the real world beyond the envelope, the new variant of the coronavirus also encounters people who are vaccinated or who have already been infected with Covid-19, slowing its spread. The doctor therefore believes that each person infected with omicron infects only three other individuals, a figure similar to that of the original virus in the Chinese city of Wuhan, which exploded onto a planet with no immune defenses and no containment measures. “With current conditions, a simple exponential growth model would still show 14 million people infected in 60 days from a single case, compared to 760,000 with measles in a population with no specific defenses,” Bhattacharyya warned.
Omicron is certainly the most rapidly spreading virus among the ones we have been able to investigate at this level of detail
Epidemiologist William Hanage, Harvard University
Historian and physician Anton Erkoreka researches epidemics from the past, and is flabbergasted by omicron’s spread. “It is the most-explosive and the fastest-spreading virus in history,” he declared. Erkoreka, director of the Basque Museum of the History of Medicine, recalls that the Black Death (14th century) and 19th-century cholera took years to spread around the world. The so-called Russian flu of 1889, which may have been caused by another coronavirus, required three months to traverse the planet. That is similar to the time taken by the original variant of SARS-CoV-2, detected in December 2019 in Wuhan and already omnipresent by March 2020. “The omicron variant has beaten that record of expansion,” Erkoreka said.
Epidemiologist William Hanage, co-director of the Center for Communicable Disease Dynamics at Harvard University, agrees. “Omicron is certainly the most rapidly spreading virus among the ones we have been able to investigate at this level of detail,” Hanage noted. One of the best-studied omicron outbreaks, a party in Oslo, Norway with a newly arrived guest from South Africa, ended with at least 81 of the 117 participants infected with the variant.
The unprecedented spread of omicron is not in doubt, but what is less clear is the effect this tsunami of cases will have on populations with high vaccination rates among the most vulnerable. In Spain, almost 100% of those aged 70 and over are vaccinated. Omicron is capable of infecting inoculated people, but vaccines still prevent serious disease, as confirmed by a recent study led by Dutch virologist Corine GeurtsvanKessel of Erasmus University in Rotterdam. The individual risk is much lower than with previous variants, but with so many infections, daily admissions to Spanish intensive care units (ICUs) are already half of those recorded during the worst peak of the wave in January 2021, when virtually no one was vaccinated.
Hanage and Bhattacharyya published a paper two weeks ago on the difficulty of ascertaining the true severity of omicron. Bhattacharyya now stresses that the new variant is perhaps 25% less severe than delta, identified in India a year ago, in people who have not been vaccinated or previously infected. “Alpha [detected in the UK in late 2020] was 50% less severe than delta according to several studies. Given the speed at which it is spreading, omicron is likely to cause a lot of damage in a shorter period of time,” warned Bhattacharyya.
Six preliminary studies suggest that omicron is able to invade the upper respiratory tract more easily, but is less effective at infecting the lungs, which would help explain its higher contagiousness and lower mortality. In research done in a lab setting, the team of virologist Michael Chan at the University of Hong Kong was the first to calculate that omicron multiplies 70 times faster in human bronchi than the delta variant but is 10 times less efficient at infecting the lungs. Microbiologist Ravindra Gupta, from the University of Cambridge (UK), has also confirmed this trend, and four other papers written by virologists from the Universities of Leuven (Belgium), Tokyo (Japan), Liverpool (UK) and Washington University in St. Louis (USA) reveal that omicron has more difficulty infecting the lungs of hamsters and genetically modified mice.
Hanage, the Harvard epidemiologist, remains cautious. Omicron seems much, much milder because citizens have built up more defenses than in previous waves, thanks to vaccination or previous contact with the virus. “Many omicron infections, especially in vaccinated people, will be mild, comparable to a short cold. The problem is that there will be serious cases and because it is so infectious, the total number that will need medical attention will put a strain on the health system,” he said. Forty percent of those hospitalized with omicron in London are unvaccinated, according to the British health authorities.
Hanage mentioned another provisional figure. “It seems that with omicron the length of a hospital stay is generally shorter and that supplemental oxygen is less likely to be needed. But it should be stressed that we have not yet seen many infections in the elderly,” he added. In Spain, the 14-day cumulative incidence is around 2,900 cases per 100,000 among people in their 20s, but for the moment it remains at 465 cases per 100,000 among octogenarians, although the figures are now soaring due to increased exposure during the Christmas holidays.
Virologist Aris Katzourakis of Oxford University (UK) also harbors doubts. “I’m still not convinced that omicron is inherently milder than the original variant that emerged in Wuhan,” he said. “It seems to be milder than delta, but this is because delta had evolved to be much more severe than its ancestor. Higher immunity seems the most logical explanation for the reduced severity seen in the population,” Katzourakis stressed.
A new preliminary study, published on December 27 by researchers at the University of KwaZulu-Natal (South Africa), suggests that omicron infection protects against the delta variant. But the study only worked with 13 subjects, mostly young and vaccinated, so it is too early to draw conclusions. The study’s lead author, biologist Alex Sigal, has acknowledged that the supposedly protective effect of omicron is not seen in the unvaccinated in his study, but his results have fueled the misconception that omicron causes a simple cold that will serve as a natural vaccine for the world. “I would love for people not to talk about omicron as if it were a live attenuated vaccine. It isn’t. Get vaccinated, get the booster and take precautions to avoid infection,” Katzourakis warned on social media.
Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization (WHO), insisted at his last press conference of the year that we do not know enough yet about omicron to lower our guard. “Omicron is spreading at a rate we have not seen with any previous variant. We’re concerned that people are dismissing omicron as mild”, he said. “Even if omicron does cause less-severe disease, the sheer number of cases could once again overwhelm unprepared health systems.”