There’s one big difference between Omicron and Delta symptoms, a major new study finds. Here’s how to tell them apart

Early impressions of Omicron, which was first detected in South Africa in November 2021, suggested that the new COVID strain differs from its Delta predecessor. Omicron infections appeared less severe, for one thing. Now, a new study of data from outbreaks of both variants in the U.K. confirms that what we already felt was right, was right.

“The prevalence of symptoms that characterise an Omicron infection differs from those of the Delta SARS-CoV-2 variant, apparently with less involvement of the lower respiratory tract and reduced probability of hospital admission,” the authors wrote. “Our data indicate a shorter period of illness and potentially of infectiousness which should impact work–health policies and public health advice.”

Using data from the U.K.’s ZOE COVID tracking app, in which users input symptoms and other information, the researchers compared 4,990 people who were infected when Delta was the predominant variant in the country (causing over 70% of infections) with an equal number infected during the Omicron wave.

The results of the study, which was published in The Lancet on Thursday, showed that while the two variants shared some common symptoms—runny nose and headaches topped the list for both—there were some striking differences. Most notably, while 52.7% of those infected with Delta lost their sense of smell, only 16.7% of those with Omicron did so.

Sore throat was more common with Omicron than Delta—70.5% to 60.8%—because Omicron stayed largely confined to the upper respiratory tract and affected a “narrower spectrum” of bodily organs.

Most importantly for people with COVID, the percentage of those infected who were hospitalized was significantly lower with Omicron than Delta—1.9% versus 2.6%, giving those with Omicron a 25% lower chance of being hospitalized.

The average duration of acute symptoms was also two days shorter for Omicron than Delta.

Omicron dominance

The WHO designated Omicron a variant of concern on Nov. 26, 2021, two days after South Africa reported the emerging mutation. Omicron quickly spread to 80 countries and soon squeezed out the previously dominant Delta strain. By Dec. 20, 2021, it was the dominant variant in the U.K.

Now, however, a new subvariant—the even more highly contagious BA.2, or “stealth” Omicron—has taken over, changing the situation yet again. Now the dominant strain in the U.S., the BA.2 subvariant is up to 60% more transmissible than original Omicron, according to the Centers for Disease Control and Prevention.

While BA.2 is widely considered to be no more severe than the original Omicron, some medical experts believe that it may be longer lasting.

“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,” Dr. David Strain, a senior clinical lecturer at the University of Exeter Medical School, who was not involved in the Lancet study, told the Guardian.

The rise of Omicron BA.2 comes as researchers have found that repeated boosters do not seem to offer a lasting increase in protection against infection (though they seem to do so against severe illness), and members of the FDA’s panel of outside advisers warn that time is running short to update vaccines for the new variants before a rise in infections expected in the fall.