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Is Omicron less virulent?

The Omicron variant of SARS-CoV-2, the virus that causes COVID-19, was first identified in November 2021 in South Africa. Early data suggested that Omicron spreads faster than previous variants but may cause less severe disease. As Omicron has spread around the world, scientists have been working to better understand the characteristics and implications of this new variant of concern.

Some key questions around Omicron’s virulence include:

Does Omicron cause less severe disease than previous variants?

Early data out of South Africa, Europe, and the US found that Omicron infections were less likely to result in hospitalization compared to previous waves driven by the Delta variant. However, there are important caveats to these findings:

– Many early Omicron cases were in young, healthy individuals who are less prone to severe COVID-19 overall.

– Two doses of a COVID-19 vaccine (and especially a booster dose) provide strong protection against severe Omicron infection. Vaccination rates were higher during Omicron waves compared to earlier pandemic waves.

– Omicron spread so quickly that it crowded out Delta circulation. Delta was shown to be more virulent than the original SARS-CoV-2 strain.

More thorough epidemiological studies controlling for these factors have generally confirmed Omicron’s lower severity, with several key findings:

– In South Africa, risk of hospital admission with Omicron was approximately 25% lower compared to the first wave dominated by an ancestral SARS-CoV-2 strain.

– In Scotland, risk of hospitalization was approximately 65% lower for Omicron compared to Delta.

– In England, risk of an emergency department visit or hospital admission was approximately 50% lower for Omicron compared to Delta.

– In the United States, risk of hospitalization was approximately 60% lower for Omicron compared to Delta.

Why might Omicron be less virulent than previous variants?

Scientists hypothesize several biological explanations for why Omicron may cause less severe disease:

– Omicron replicates faster in the upper airways but less efficiently in lung tissue compared to earlier variants. This could lead to less pneumonia and severe lung infections.

– Omicron infection leads to less inflammation and immune system hyperactivity that often precipitates severe COVID-19.

– Omicron is less able to infect lung cells expressing the ACE2 receptor compared to variants like Delta.

– Omicron is more rapidly cleared by the immune system.

However, the exact mechanisms behind Omicron’s reduced virulence are still under investigation.

Does Omicron still pose risks for vulnerable populations?

Despite being less virulent on average, experts caution that Omicron still poses significant risks of severe disease and death, especially for unvaccinated, older, and medically vulnerable populations.

Some key points:

– Unvaccinated individuals remain at high risk of severe Omicron infection. Vaccines provide substantial protection against hospitalization and death.

– People over 60 still account for a disproportionate share of Omicron hospitalizations and deaths. Age remains a significant risk factor.

– Those with medical comorbidities like obesity, diabetes, immunodeficiencies, chronic lung, and heart conditions also face elevated risks with Omicron infection.

– Omicron spreads so quickly that even a fraction of cases needing hospital care can overwhelm healthcare systems.

So while Omicron represents a clear reduction in severity compared to prior variants on a population level, it remains vital for vulnerable individuals to get vaccinated and take appropriate precautions to avoid infection.

Impact on future pandemic waves

Assuming Omicron remains the dominant global variant or shapes future SARS-CoV-2 evolution, its reduced virulence could have important implications for the trajectory of the pandemic.

Lower hospitalization and death rates

All else being equal, a less virulent prevailing variant should lead to fewer hospitalizations and deaths per infection. However, the sheer transmissibility of Omicron can offset this benefit, especially in unvaccinated populations.

In South Africa, Omicron sent hospitalizations skyrocketing despite its lower severity, putting a massive strain on healthcare capacity. Vaccination coverage there was only around 30% of the population at the time, leaving many vulnerable.

In countries with higher vaccination rates like the UK and Canada, overall hospitalizations and deaths remained well below previous pandemic peaks during Omicron waves, although pressures on healthcare systems were still considerable.

Faster transition to endemicity

Some experts suggest that viral evolution towards greater transmissibility but lower virulence is a common pathway for viruses to become endemic in human populations.

Endemic viruses circulate constantly without causing large epidemics. The four existing endemic human coronaviruses, for example, produce mild cold-like symptoms in most cases.

If Omicron or its descendants displace more virulent variants, COVID-19 could begin transitioning from a severe pandemic disease to a persistent but manageable endemic illness over the next 1-2 years. However, the emergence of yet another highly transmissible and virulent variant remains possible as well.

Challenges for achieving herd immunity

Herd immunity occurs when enough of the population has immunity, reducing viral transmission overall.

Some thought achieving high levels of immunity through vaccination and infections would end the pandemic. But Omicron’s increased immune evasiveness has drastically reduced post-infection and vaccine-induced immunity.

Omicron outbreaks in countries that approached herd immunity levels against Delta like Portugal and Denmark underscore how population-level immunity has been partially reset.

With Omicron more likely to reinfect, achieving durable herd immunity may be more difficult unless vaccines are updated to restore protection against infection.

Need for updated vaccines

Vaccines still provide good protection against severe Omicron illness, but less so against infection and transmission. Omicron-targeted vaccine boosters will likely be necessary to restore that protection against milder breakthrough cases.

Moderna, Pfizer, and other companies are testing Omicron-specific booster formulas. These could become part of annual fall COVID vaccine campaigns if Omicron remains dominant.

Updated boosters also ideally need to elicit mucosal immune responses blocking infection in the nose and upper airways to further curb transmission.


In summary, analyses conducted to date consistently show Omicron is less clinically virulent than previous SARS-CoV-2 variants, especially in vaccinated populations. However, it still poses substantive risks that require ongoing precautions and vaccination. Looking forward, Omicron’s properties may contribute to COVID-19 transitioning from a pandemic to a milder endemic disease over time. But its increased immune evasion underscores the need for updated vaccines targeting currently circulating variants. While Omicron clearly marks a positive turn towards less severe COVID-19, the pandemic endgame remains uncertain as the virus continues evolving. Careful tracking of emerging variants and tailoring protective measures accordingly will be vital to navigating the next phases.

Variant Transmissibility Disease Severity
Original Baseline Baseline
Alpha Higher Similar
Delta Higher Higher
Omicron Much higher Lower

Key points

  • Omicron causes less severe disease compared to prior variants, especially Delta
  • However, it remains dangerous for unvaccinated and vulnerable groups
  • Omicron’s properties may enable a gradual endemic transition
  • But immune escape reduces population immunity
  • Updated vaccines are likely needed to restore protection