Is the Omicron COVID variant less severe than Delta?

Since emerging in December 2021, Omicron has spread in over 80 countries and has become the dominant COVID-19 variant worldwide, overtaking the Delta variant with symptoms that are much more similar to that of a cold. 

What did we find?

Thanks to contributions to the ZOE COVID Study app, we’ve been able to conduct one of the largest worldwide population studies into how the symptoms of Omicron compare with those of Delta.

We found that people with an Omicron infection are more likely to have a sore throat and are less likely to experience loss of smell, compared with Delta. 

Our researchers from both ZOE and King’s College London also found that the Omicron variant is associated with a lower risk of hospitalization and shorter duration of illness in the vaccinated population than previous variants. 

How did we conduct the research?

This latest study was published on the 7th April in The Lancet. The research looked at symptoms of 62,002 vaccinated participants in the U.K. from the ZOE COVID Study App who tested positive between June 1–November 27, 2021, when Delta was dominant, as well as from December 20, 2021–January 17, 2022, when Omicron was dominant.

How are the symptoms different?

The findings show that the duration of COVID-19 symptoms was significantly shorter (6.87 days versus 8.89 days), and participants were less likely to be hospitalized with the Omicron variant compared with the Delta variant.

The most striking difference between variants was the difference in loss of sense of smell — a common symptom of earlier variants — which is now only appearing in under 20% of cases and often days after the first appearance of symptoms. 

The two symptoms that were consistently more prevalent among Omicron than among Delta cases (regardless of vaccination status) were sore throat and a hoarse voice.

Many other more serious symptoms — like brain fog, eye soreness, fever, and headaches — were all significantly less prevalent in Omicron cases, though they can still occur.

Researchers note that Omicron is found far less frequently in the lower respiratory tract, where infection can cause more severe symptoms, sending people to the hospital, and that Omicron symptoms do not last as long in vaccinated people.

Who is affected by this?

While everyone still has a risk of catching COVID, we found that symptoms of an Omicron infection don’t last as long in vaccinated people as in the unvaccinated population. 

This is further evidence to suggest that vaccination, in spite of being developed prior to Omicron, still helps to prevent long-lasting symptoms in those who are infected.

With a shorter duration of symptoms in the vaccinated population, this suggests that the incubation time and period of infectiousness for Omicron may also be shorter.

This could impact future workplace health policies and is likely to have been key to the public health guidance shared by the U.K. prime minister regarding dropping of isolation restrictions.

Why are Omicron symptoms less severe?

While further research is needed, Omicron is likely less severe than Delta — both in terms of symptom severity and number of hospitalizations —  because Delta is better at infecting lung cells than Omicron, which enters other mucosal cells. 

Although Omicron appears to be much more transmissible than previous variants, our studies indicate that Omicron causes fewer severe symptoms in multiple organs that are more affected by Delta.

In addition, a very recent in vitro study showed that while Omicron replicates faster than all other COVID variants in the upper respiratory tract (which is made of similar cells to those that are found in the throat), this variant is extremely inefficient at replicating in the the alveoli deep in the lungs.

As such, Omicron appears to enter human cells by a different route than other variants.

Diffuse alveolar damage (damage to this particular tissue in the lungs) is the primary form of severe lung disease in COVID-19 patients. Such in vitro data are consistent with the lower severity of symptoms seen in our study.

What the scientists have to say

Dr. Cristina Menni, from King’s College London, said: 

“We observe a different clinical presentation of symptoms in those infected with Omicron compared to Delta. As we are moving even further away from the average patient having U.K. government “approved symptoms,” i.e. fever, persistent cough, loss of smell, our results point to a different selection of symptoms that may indicate infection. To protect others, it is still important to self-isolate for 5 days as soon as you see any symptoms.”

Professor Ana Valdes, an honorary professor at King's College London, added: 

“Although there is still a wide range of duration and severity of symptoms with Omicron, for vaccinated individuals we find on average a shorter duration of symptoms. This suggests that the incubation time and period of infectiousness for Omicron may also be shorter, which has now impacted workplace health policies and public health guidance.”

What does this mean for me?

Although Omicron symptoms are less severe than Delta or other previous COVID-19 variants, there may still be risks of long COVID or illness, especially in the more vulnerable population.

With the restrictions relaxing globally, it’s still a good idea to practice responsible citizenship and avoid spending time in places where you might pass on the virus to others, just as you would with any normal cold or flu.

We recommend getting a test if you’re feeling unwell and trying to limit your social contact with those who might be more affected by an infection.

We still want you to log all of your tests and symptoms, as COVID is still with us. The app is a crucial tool to track the symptoms and spread of COVID.

It’s thanks to the loyal contributors to the ZOE COVID Study that the teams at ZOE and King’s College London can do this essential research so rapidly, before anyone else in the world. 

So, if you are one of those loggers, thank you! And if you want to get involved, it’s never too late to Join ZOE! 

Stay safe and keep logging!