New Covid-19 infections are reaching record levels in the U.S. and Europe. The surge is due both to the Delta variant that flourished over the past six months and to the new Omicron variant, which, in the week of December 19–December 25, became predominant, accounting for approximately 60 percent of U.S. cases. The rise of Omicron has largely been greeted with apprehension, but the new variant, which tends to cause mild illness, offers hope for a more promising path forward.

A preprint report from South Africa, where Omicron first spread in November, found that individuals infected with the variant had seen increased neutralizing immunity against Delta, making it less likely that Delta would re-infect them. This creates the possibility that large numbers of Omicron cases could immunize the population against past variants and perhaps future ones. Omicron is likely more transmissible than Delta, and Delta has been estimated to be about twice as transmissible as earlier variants. Omicron seems to replicate faster and in greater quantities than earlier variants, meaning that infected people release more virus to infect others. It also appears to be better at evading neutralizing immunity elicited by vaccines or previous infections with earlier variants, including Delta. Omicron was associated with a fivefold higher risk of reinfection and a two-to-four times higher risk of post-vaccination breakthrough compared with Delta in an English study.

These features suggest that Omicron will infect large numbers of people, including many with vaccine and natural immunity. But again, most of these cases will be mild since Omicron by all indications so far seems to be less severe than earlier variants.

Analyses from the Imperial College London, and from Scotland and South Africa, indicate that those infected with Omicron had a 45 percent to 80 percent lower risk of hospital admission than those with a Delta infection. A new analysis by the U.K. Heath Security Agency that adjusts for risk factors including age, sex, and vaccination status found that the risk of hospitalization with Omicron was about a third of the risk associated with Delta. And once hospitalized, Omicron patients fare better than patients with earlier variants.

A just-published study in the International Journal of Infectious Diseases found that 4.5 percent of Covid-19 patients admitted to a large South African hospital from November 14, 2021 and after (the Omicron period) died, compared with 21.3 percent deaths for admissions in the pre-Omicron period. During the Omicron period, 1 percent needed ICU admissions versus 4.3 percent in the earlier period, and the length of hospital stay was cut in half—4.0 days versus 8.8 days. Sixty-three percent of the admissions were “incidental COVID admissions”—patients admitted for another serious medical problem who tested positive for Covid-19 on routine testing—suggesting high levels of asymptomatic or mild illness with Omicron.

Another new study, in JAMA, of 49 South African hospitals comparing the current Omicron wave with the three previous Covid waves found similar results. Only 2.7 percent of hospitalized, Omicron-wave patients died—about one-tenth the proportion of earlier waves (19.7 percent to 29.1 percent). They were about half as likely to be admitted to the ICU, between four to ten times less likely to need mechanical ventilation, and their median length of stay (three days) was two-thirds shorter compared with earlier waves.

Omicron is currently displacing other more virulent strains. Even if it quickly peaks and fades away—the South African Omicron wave started to decline within four weeks—Omicron’s spread has been so rapid and widespread that it will lead to many people having mild disease, while developing immunity to subsequent infection by earlier variants, including Delta. Moreover, it is possible that widespread Omicron infection could help limit transmission of future variants. Omicron has far more mutations than earlier variants—more than 30 on the spike protein that is crucial for all variants to attack human cells. If, as the South African study suggests, the highly mutated Omicron spike protein elicits immunity to earlier variants such as Delta, then it seems likely it would provide protection against future variants, too.

This expansion of natural immunity will be especially important in countries with low levels of vaccination. Omicron is spreading worldwide, and the resulting immunity could limit the ongoing circulation of the virus and the emergence of new variants.

U.S. Omicron infections are likely to peak and start declining within the next few weeks. While most cases will be mild, the large numbers of cases will increase hospitalizations, straining capacity in some locations. But this should be short-lived, with only small increases in deaths. With luck, the surge of this highly contagious variant can provide widespread immunity and help hasten the end of the pandemic.

Photo by Spencer Platt/Getty Images


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