Omicron Covid symptoms less severe than Delta variant and period of infectiousness is shorter, study finds

Symptoms of Omicron will typically be milder and the risk of hospitalisation less by comparison to the Delta Covid variant

People who are infected with the Omicron Covid variant are likely to suffer with different symptoms to those who contract Delta, a new study has found.

Research indicates that symptoms will typically be milder, and the risk of hospitalisation less likely, with the current dominant strain of coronavirus.

Those who have been vaccinated against Covid are also likely to suffer with symptoms for a shorter time with Omicron compared to the Delta strain, with people typically feeling unwell for 6.87 days versus 8.89 days respectively for the two variants.

The findings support previous studies that suggest the incubation time and period of infectiousness for Omicron is shorter than previous variants of coronavirus.

A sore throat and hoarse voice are common of both the Omicron and Delta variants (Composite: Kim Mogg / JPIMedia)A sore throat and hoarse voice are common of both the Omicron and Delta variants (Composite: Kim Mogg / JPIMedia)
A sore throat and hoarse voice are common of both the Omicron and Delta variants (Composite: Kim Mogg / JPIMedia)

How do symptoms differ between Omicron and Delta?

The study found that people who contract Omicron are more likely to have a sore throat and less likely to lose their sense of smell compared to those infected with Delta.

The biggest difference between the two variants was the loss of smell, which appeared in 52.7% of Delta cases, but showed up in less than 20% of Omicron cases.

The two Covid symptoms found to be consistently prevalent in both variants, regardless of vaccination status, were a sore throat and hoarse voice.

Researchers also found that some of the more debilitating symptoms were much less common in Omicron cases. These include:

  • brain fog
  • eye soreness and burning
  • dizziness 
  • fever 
  • headaches 

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 UK government ‘core’ 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 five days as soon as you see any symptoms.”

Researchers from King’s College London and scientists from Zoe studied the symptoms of 62,002 vaccinated UK participants from the Zoe Covid Study App who tested positive between 1 June last year and 27 November 2021, when Delta was dominant, and 22 December 2021 to 17 January this year when Omicron was dominant.

The study will be presented at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) this month in Lisbon.

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.”

What are the key Covid symptoms?

The NHS recently added nine new Covid symptoms to its official list more than two years into the pandemic, extending the list from just three to 12 in total.

The three traditional symptoms of a fever, a new and persistent cough, and a loss or change in taste or smell are still included, with people now advised to also look for the following signs:

  • shortness of breath
  • feeling tired or exhausted
  • an aching body
  • a headache
  • a sore throat
  • a blocked or runny nose
  • loss of appetite
  • diarrhoea
  • feeling sick or being sick

Both the World Health Organisation and the Centres for Disease Control and Prevention in the US have had longer symptom lists for some time, bu the UK list featured just three symptoms for almost two years.

Professor Tim Spector, lead scientist of the Zoe Covid-19 symptom tracker app, welcomed the update after being highly critical of the government’s “refusal” to recognise a “wider array of symptoms”.

However, he warned that the list still places too much emphasis on symptoms like fever and loss of smell or taste when findings have shown Omicron is more likely to cause more cold-like symptoms, such as a runny nose or sneezing.

He said: “The addition of more symptoms is definitely a step in the right direction and it could help reduce infections as we go forward.

“However, whilst this is good news, I’d like to see the order of the symptoms changed, as the NHS list puts far too much emphasis on symptoms like fever, and anosmia, which we know are much less common since the Omicron variant emerged.

“According to the Zoe Covid Study, the top five symptoms being reported by contributors with a positive Covid test are runny nose, fatigue, sore throat, headache and sneezing.”