The dominant strain of Covid-19 in the UK is now Omicron - but there is still a sizeable presence of the Delta form of the virus, too.
The two variants also do present different symptoms, a new report from the UK Health Security Agency says.
Analysis from the UKHSA looked at the symptoms reported to NHS Track and Trace by those who tested positive for Covid in December, Wales Online reports.
Confirmed cases were asked if they displayed any of the following symptoms: fever, cough, shortness of breath, fatigue, altered consciousness, muscle or joint pain, headache, loss of smell or taste, sore throat, runny nose, sneezing, rash, red or irritated eye, loss of appetite, nausea or vomiting, or diarrhoea.
The average number of reported symptoms was the same for both variants at four.
A sore throat was more likely to be reported by cases with Omicron (53 percent of Omicron cases, 34 percent of Delta cases).
On the other hand, loss of smell and taste was found to be less common among Omicron compared to Delta cases (13 percent of Omicron cases, 34 percent of Delta cases).
Analysis was based on 182,133 Omicron and 87,920 Delta cases reporting symptoms with onset between December 1 and December 28, who completed contract tracing and reported symptoms, and had provided data on their age, sex and region.
As the graph below shows, when adjusted for these and other factors, people with Omicron are much more likely to report a sore throat than those with Delta, and much less likely to report a loss of taste and smell.
The adjusted odds ratio analysis compared the odds of a specific symptom being reported by an Omicron or a Delta case whilst adjusting for age group, sex, ethnicity, self reported vaccination status (two or more doses, one or no dose, or missing data), geographical region of residence, and the week in which symptoms began.
However, one of the limitations of this analysis was that it didn’t include symptoms among those who tested negative for Covid, so couldn’t give an indication of how specific the symptoms are to a Covid infection.
A recent study led by Oxford University and the Office for National Statistics studied symptoms reported by PCR-positive and PCR-negative individuals using data from the UK Covid-19 Infection Survey.
As Omicron cases increased as a proportion of SARS-CoV-2 infections during December 2021, the study found increased reports of sore throat and a marked reduction in reporting of loss of smell and taste in PCR-positives.
However, this study also found that sore throat became more commonly reported in symptomatic PCR-negative cases during this period, suggesting that sore throat may not be a specific predictor of SARS-CoV-2 infection.
Another limitation was that symptom data was collected at the time of contact tracing only (usually 3-4 days post symptom onset), and therefore additional symptoms presenting at a later time are not captured.
Time taken between symptom onset and completing contact tracing was slightly longer for Omicron cases compared to Delta.
