Which virus causes yellow fever

Reports have varied. That shift began with the Delta variant and has remained true of Omicron, says Tim Spector, a genetic epidemiologist who founded the consumer health company ZOE, which runs a COVID symptom tracking app to which more than 4. Other research has come to slightly different conclusions. And a small study from Norway found that, among people in one case cluster, a cough was the most common symptom associated with the variant, followed by runny nose and fatigue.

Like ZOE, the Norwegian researchers also observed a significant decrease in smell and taste loss. Data from early Omicron hotspots including South Africa, the U. More research is needed to determine how often this happens. Despite signs that Omicron leads to a lower individual risk of severe illness, it's so contagious that hospitals are struggling to handle the growing caseload.

A consortium of disease forecasters warned Tuesday that the U. Scientists caution that Omicron's rapid ascent may be fueled in large part by more people spreading the virus with no symptoms at all.

One study of health care workers in South Africa, which has not yet been peer-reviewed, found the rate of people carrying the disease without symptoms had climbed from 2. While the frequency of some symptoms may have changed with Omicron, doctors say there is no evidence of significantly different or new symptoms that have not been seen in previous COVID cases. Instead, scientists will need to sort out how Omicron may have shifted the odds of some symptoms compared to others.

For example, loss of smell or taste — called "anosmia" and "ageusia," respectively, by doctors — emerged early during the pandemic as a telltale sign of COVID However, estimates of how many cases actually result in the symptom have ranged widely over the course of the pandemic and between different parts of the world. In November, one study estimated that as many as 1. Night sweats, another symptom cited by a doctor at a media briefing in South Africa early in the country's Omicron wave, has been touted by tabloids as "a telltale sign which only happens at night that could mean you have been hit by the Omicron variant.

Since June, monthly government surveys in the U. A new round of survey data for December, when the Omicron variant began to drive a record spike in cases there, is scheduled to be released later this month. New clues could also come from ongoing studies investigating Omicron's effects on animals and tissue samples challenged with the virus in labs, which can control for factors like immunity or underlying conditions that might muddy data on the variant's effects in humans. Yellow fever vaccination.

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