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How is Omicron outcompeting Delta so quickly? Six possibilities to consider

Daniel R. Lucey, MD, MPH, FIDSA
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At today’s WHO COVID-19 press conference (minute 2:25-2:42), the WHO Director-General, Dr. Tedros Adhanom Ghebreyesus, stated that Omicron is in at least 77 nations and is spreading faster than any prior COVID variant. The UK, Denmark and Norway are reporting that within a week Omicron could beat Delta in terms of the percentage of the population infected. Omicron has been reported in 30 U.S. states. Here in New York City, community transmission has been reported. China reported its second Omicron case today in a man flying home to Guangzhou from Shanghai after 14 days in quarantine, during which he tested negative. Thus, Omicron community transmission might have begun in China less than 2 months before the Olympics (Feb. 4) and the start of the Lunar New Year of the Tiger (Feb. 1).

How is Omicron outcompeting Delta so quickly? Hypotheses (alone or in combination) include:

Immune evasion? Neutralizing antibodies both natural and vaccine induced, possible evasion of innate immunity, and less likely of virus-specific CD8+ T cells.

Higher and/or earlier viral load peak? Delta was previously reported to have higher viral load than Alpha. Thus, Omicron might have even higher viral load still, especially if it can evade initial innate immunity (e.g., interferons, NK-cells, specific cytokines and chemokines, others) as well as neutralizing antibody. (There is no evidence of antibody-mediated enhancement of infection.)

More airborne? Months ago, the spread of Delta was reported by the U.S. CDC to be more like chickenpox (varicella) virus in terms of airborne spread (droplet nuclei). Is Omicron even more airborne, moving it closer toward somewhere between chickenpox and measles virus? How long can Omicron stay suspended in the air compared with Delta or Alpha or the original virus found in 2019 in Wuhan or the early 2020 D614G virus?

More asymptomatic spread? Today a systematic review from China on asymptomatic spread (pre-Omicron) published in JAMA Network Open emphasized once again the importance of asymptomatic spread in this pandemic. Thus, it is reasonable to assess whether Omicron has even more asymptomatic or presymptomatic transmission than Delta, Alpha or 2019 and early 2020 versions of this virus.

Shorter Incubation period? If so, then infected persons could become contagious (even if asymptomatic) faster and transmit faster than those infected with Delta.

Mechanism X? This is my neologism to include mechanisms that are unknown.


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