Omicron variant does cause different symptoms from Delta, study finds

People who have the Omicron Covid variant tend to have symptoms for a shorter period, a lower risk of being admitted to hospital and a different set of symptoms from those who have Delta, research has suggested.

As the highly transmissible Omicron variant shot to dominance towards the end of last year, it emerged that, while it is better at dodging the body’s immune responses than Delta, it also produces less severe disease.

Now a large study has not only backed up the findings, but confirmed reports Omicron is linked to a shorter duration of illness and a different collection of symptoms.

The study comes just days after the NHS added nine further symptoms for Covid to its existing list of fever, a new and persistent cough, and a loss or change in taste or smell. The researchers found people who had Covid when Omicron was prevalent were about half as likely to report having at least one of the latter three symptoms as those who had Covid when Delta was rife.

“It is a lesson that we need to be far more flexible in thinking what the virus is and how it is going to present than we have been, certainly in the UK,” said Prof Tim Spector, co-author of the research from King’s College London, adding that the team showed data to the government around five months ago that showed a sore throat was replacing loss of smell as a symptom.

The study, which is to be presented at the European Congress of Clinical Microbiology & Infectious Diseases and has been published in the Lancet, is based on data from 63,002 participants of the Zoe Covid study.

The researchers matched almost 4,990 participants who had a confirmed Covid infection between 1 June and 27 November 2021 – when Delta was the most prevalent variant – with 4,990 individuals who reported an infection between 20 December 2021 and 17 January 2022 when Omicron dominated, with the matching based on age, sex and whether they had received two or three vaccine doses. The experience of both groups were then compared.

The team found participants’ symptoms lasted on average 6.9 days during the period when Omicron dominated, compared with 8.9 days when Delta dominated, with infections during the Omicron period linked to a 25% lower likelihood of admission to hospital.

The results suggested only 17% of people who had Covid when Omicron dominated lost their sense of smell, compared with 53% when Delta dominated. However, a sore throat and going hoarse were both more common among the former.

Spector said the symptom-logging approach used in the research was an invaluable tool. “[It] should alert us what to look out for when there will inevitably be the next variant,” he said, adding action needs to be quicker in the future.

“We need to be much more reactive in public health messaging,” he said.

Dr David Strain, a senior clinical lecturer at the University of Exeter Medical School, who was not involved in the study, said the findings chime with what hospitals experienced at the start of the year when the BA.1 Omicron variant dominated. But the Omicron variant BA.2 has since taken over – and, in his experience, the picture has changed again.

“People in hospital are staying in hospital for longer and staff are testing positive for longer, so it is longer before they can return to work,” he said.

The research came as the React-1 study revealed the average prevalence of Covid across England was at the highest level ever recorded. Meanwhile, the UK Health Security Agency reported on Thursday that 15.3% of people aged 75 and over have received their spring booster Covid jab so far.

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