Since the COVID-19 virus first appeared in December 2019, thousands of variants have emerged — but, until this month, Delta reigned supreme.
The arrival of Omicron — with the first U.S. case detected in San Francisco on Wednesday and more in New York, Minnesota and Colorado a day later — is a reminder that Delta isn’t the endgame.
How do COVID-19’s two most recent and worrisome variants compare? Here’s a snapshot of what we know.
Mutations
Omicron has the most mutations of any variant of SARS-CoV-2, the virus that causes COVID-19. We still have to learn how those genetic changes behave. But they take hold for a reason — because they help a variant excel.
Delta: It has 13 mutations. Of these, nine are in the spike protein, the protrusion on the surface of the virus that helps it latch onto human cells. Specifically, two are in a molecular hook, called the “receptor-binding domain,” helping it cling to cells more tightly, according to Suresh V. Kuchipudi of Penn State University, who studies viral evolution.
Omicron: It’s a mutation monster — at least 32 are in the spike protein and 10 in the receptor-binding domain.
While it kept many of the most successful mutations found in earlier variants, including Delta, it also possesses changes found nowhere before.
What are all of these mutations doing? There’s not enough information yet to say whether they’ll make omicron worse than Delta. What really matters here is the combination of mutations and not any single mutation on its own, according to Kristian Andersen, an infectious disease researcher at Scripps Research.
But here’s a clue: Because so many are on the spike protein, they may enhance the variant’s ability to infect cells and evade immunity.
Contagion
As the world discovered with Delta, higher transmissibility puts unvaccinated individuals at great risk.
Delta: On average, the original SARS-CoV-2 virus spread from one person to two or three. Delta changed that — infecting about six people. And Delta has an incubation period of only four days, faster than the six days seen in the original virus, so people are more quickly contagious.
Omicron: We don’t yet know its transmission rate or incubation period. We’ll know more about the variant’s growth rates in different geographies within the next few weeks.
But the rapid increase in cases across South Africa is concerning — and suggests that Omicron is outcompeting Delta in that nation, said Trevor Bedford, an expert on viral evolution and surveillance at the Fred Hutchinson Cancer Research Center in Seattle.
That does not necessarily mean that Omicron is intrinsically more transmissible than Delta. Instead, It could be better at escaping our immune defenses.
Immune escape
A variant that can evade our antibodies puts previously infected and vaccinated individuals at risk. But that’s not devastating news, because vaccines can be re-designed to be more protective.
Delta: This variant acquired some mutations that allows it to evade vaccine-induced antibodies — which explains, in part, the problem of vaccine “breakthroughs.”
But there’s been little evidence of increased reinfection risk associated with Delta. The antibodies created by older strains were effective in protecting people.
Omicron: It shares similar mutations with the earlier Beta and Gamma variants, which are more resistant to vaccination than Delta. We’ll know much more about this risk in several weeks, when lab work is done. Scientists are particularly interested in comparing Omicron’s fate against the antibodies made by people who only got two doses of vaccine — versus those who got two doses, plus a booster.
But the Omicron infection in a vaccinated San Francisco resident suggests that two doses, alone, won’t protect us. Additionally, a newly released study of epidemiological data from South Africa shows a three-fold increase in risk for reinfection due to Omicron, when compared to Delta.
Emergence
Delta: First identified in India in December 2020, Delta is presumed to have come from someone whose immune system was suppressed, perhaps due to medications or an illness such as HIV/AIDS.
Omicron: Based on the branching of its genetic tree, experts believe Omicron is relatively young, with most estimates placing its emergence in mid-October. According to the journal Science, Omicron did not develop out of one of the earlier variants of concern, such as Alpha or Delta. Instead, it seems to have evolved in parallel — and Omicron is so different from other variants that pinpointing its closest relative is difficult, according to the journal Science.
Severity of illness
Andersen calls this “the million dollar question.”
Delta: A report this summer found that people in England with Delta had double the hospitalization risk of those with an earlier variant. And Delta killed huge numbers of people simply because it infected so many more.
Omicron: We don’t yet know about Omicron’s virulence. In South Africa, cases are rising faster than hospital admissions — a hint of less severe disease. And even though hospital admissions have increased, so far there is no increase in use of ICU beds.
That may be because cases of Omicron, so far, are in predominantly younger people. It could also suggest that in vaccinated and previously infected people, the body’s “memory” immunity is kicking in.
But if reinfections or “breakthrough” infections cause hospitalizations, that’s bad news. It suggests our antibodies aren’t protective.
Scientists are keeping a close eye on what happens to cases and hospitalizations over the next few weeks, according to Andersen — especially in places like the U.S., with a medium vaccination rate; the U.K., with a high vaccination rate; and Israel, with one of the world’s highest vaccination and booster rates.
