Is it true that Omicron is less severe than previous variants?

COVID Variants

A: We HOPE so, but we don’t know yet. The evidence so far is mixed.


We are WAY past due for some good variant news. But pinning our Omicron hopes on a less virulent variant is not wise for two reasons:

1) It might not be less severe in those who are “immune naïve” (neither vaccinated nor with a previous infection—still millions of people).
2) Many more infections *even* if less severe can still lead to an avalanche of hospitalizations and deaths, and the risk of long Covid.

The rumors about Omicron were flying fast and furious even before it was granted its Marvel-character-sounding Greek letter.

“It spreads faster.” “The vaccines don’t work for it.” “It’s less virulent.”

Data is finally starting to come in to put these rumors to the test.


Omicron does indeed seem to be spreading faster than Delta, meaning it has a “growth advantage.” What we don’t know yet is how much of this is due to Omicron being inherently more transmissible vs. better at infecting people with prior immunity (probably a bit of both).

*Vaccine efficacy*

So far numerous lab-dish neutralization studies *and* some real-world data suggest Omicron is better at evading antibodies generated from 2 doses of vaccines or previous infection, but 3 doses hold up well. Get boosted!


➕ So far, we have conflicting evidence on this one. Early data from South Africa was largely anecdotal—Doctors reported that the Omicron cases they were seeing were mild. This isn’t our favorite type of data because it could be down to the luck of who the doctor on TV happened to see, or because young people were the first to get Omicron in South Africa. As more data have come in, there does still appear to be a lower rate of hospitalization in South Africa for Omicron compared to previous waves.

Even if this conclusion holds up, this decreased severity in South Africa could be a result of higher levels of population immunity (from natural infection + vaccination) compared to previous waves rather than any intrinsic characteristic of Omicron. South Africa has already been hit *extremely* hard by previous waves of COVID-19, with excess mortality estimated at 230,000 deaths out of a population of 59 million, meaning much worse per capita than even poor performers like the U.S, U.K. and Brazil. If Omicron infects more people who have some level of previous immunity, it will look less severe overall even if the severity hasn’t changed for people who are “immune naïve” (see figure).

South Africa is also a much younger country compared to Europe or North America. This all means that we can’t generalize too much from the experience of one country, as the impact on countries with different infection+ vaccination+ age patterns could be different.

➕Two recent studies in lab culture showed that Omicron may be less efficient at replicating in lung tissue compared to the upper respiratory track. Many scientists were quick to point out that disease severity is a much more complicated interaction between the virus and immune response of the host, which does not necessarily correspond to viral replication in a lab dish. So, this evidence shows nice biological plausibility but is not a slam dunk.

➖Recent data from the UK shows “no signal” of lesser severity for Omicron. This is concerning but still based on small numbers given the lag time needed to identify hospitalizations and the newness of the variant. Hospitalizations in London, which has seen the earliest Omicron spike, were up 42% week-on-week, perhaps a forecast of what is to come.

➖Finally, a variant that is twice as transmissible, even if it were half as severe can still lead to the same number of hospitalizations in a short time. So, while this trade-off might be good in the long-term, it’s still very dangerous in the short term when health systems are overwhelmed, too many people are still unvaccinated, and we are running short on effective treatments such as monoclonal antibodies.


We REALLY HOPE Omicron causes less severe disease. But we don’t yet know. And “hope is not a strategy.”

*By the time we have good evidence of Omicron’s severity, it will be too late to react. We should all take precautions now to limit spread and boost our immunity through vaccination.

*Countries with high vaccination rates like the UK saw much lower hospitalization and death rates with Delta compared to previous waves, so we know this can be done.

*Whether Delta or Omicron, lower severity doesn’t necessarily mean the virus is weaker, but that WE (through immunity) are stronger. #BoostUp

Those Nerdy Girls


“Omicron may be less severe in South Africa. That may not be the case for the U.S.”

UK Data on Omicron

Studies of Omicron in lung tissue:


Gupta Lab

Figure Source: “Challenges in inferring intrinsic severity of SARS-CoV-2 Omicron variant from early population-level impact.”

Note for Figure: Delta (left) swept South Africa in June-August 2021, when population immunity was lower. By contrast, Omicron (right) encountered a population with fewer non-immune individuals (orange), and can more readily infect immune individuals (green). Omicron is expected to infect many more individuals at low risk for severe outcomes due to prior immunity, which will artifactually reduce observed severity (infection fatality rate/IFR).

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