In Boulder, Colorado, people ages 18 to 22 will soon be banned from all gatherings, whether indoors, outdoors, or with people outside that age range, the Boulder County Public Health Department announced.

This comes after nearly three weeks of cases rising in Boulder County, "with the vast majority" connected to 18 to 22 year olds, "particularly those attending the University of Colorado Boulder," the department said.

"While there has been little evidence of spread of the virus beyond the 18-22 age group, the continued increase in new cases in the city of Boulder prompted the Public Health Order to help prevent the spread to other community members," the department said.

The health department also calls out 36 addresses where the department claims residents "repeatedly engaged in activities that violate public health orders." Those residents must stay in their homes for two weeks, only leaving for essential services, according to the Health Department.

The order will last for two weeks, beginning at 4 p.m. local time on Thursday.

Residents in the 18 to 22 age group are exempt for exercising outside alone, studying outside alone and going to work if they don’t live in one of the homes with stay-at-home requirements.

Young adults can interact with relatives they live with, and those in high school can participate in school activities, the department added.

"While there has been little evidence of spread of the virus beyond the 18-22 age group, the continued increase in new cases in the city of Boulder prompted the Public Health Order to help prevent the spread to other community members," the department said.

ABC News' Jennifer Watts contributed to this report.

The United Kingdom reported 6,634 daily COVID-19 cases on Thursday, reaching a new record high.

The number of hospital admissions is also on the rise.

The U.K. now has over 412,000 coronavirus cases and at least 41,951 fatalities, according to Johns Hopkins data.

ABC News' Mike Trew contributed to this report.

The United Kingdom’s long-awaited contact tracing app launched in England and Wales on Thursday.

The app, available to download for free on iPhones and Androids, alerts users if they've come into contact with anyone diagnosed with COVID-19.

"The app will advise you to self-isolate if you have been in close contact with a confirmed case," the Department of Health and Social Care said. "It will also enable you to check symptoms, book a free test if needed and get your test results."

The BBC reported that over 1 million people have already downloaded the app.

ABC News' Mike Trew contributed to this report.

Africa has seen a steady decline in the number of new COVID-19 cases being reported across the continent since July 20, the World Health Organization said Thursday.

Over the past four weeks, there were 77,147 new cases recorded in Africa, down from 131,647 reported in the previous four weeks. Some of the continent's most-affected countries including Algeria, Cameroon, Cote d’Ivoire, Ethiopia, Ghana, Kenya, Madagascar, Nigeria, Senegal and South Africa have all seen infections drop every week over the past two months. Deaths attributed to COVID-19 have also remained low in the region.

Africa was the only region in the world to report a decline in both new COVID-19 cases and deaths for the week of Sept. 14-20.

The WHO credited the decline to "a variety of socio-ecological factors as well as early and strong public health measures taken by governments across the region." The socio-ecological factors include low population density and mobility, hot and humid climate and lower age group.

"The downward trend that we have seen in Africa over the past two months is undoubtedly a positive development and speaks to the robust and decisive public health measures taken by governments across the region," Dr. Matshidiso Moeti, the WHO's regional director for Africa, said in a statement Thursday. "But we must not become complacent. Other regions of the world have experienced similar trends only to find that as social and public health measures are relaxed, cases start ramping up again."

Africa's COVID-19 cases have largely been among a younger age group and more pronounced in a few nations rather than continent-wide, suggesting country-specific aspects are driving the pattern of disease and death. About 91% of infections in sub-Saharan Africa are among people below the age of 60, while over 80% of cases are asymptomatic, according to the WHO.

Although African governments have made efforts to improve COVID-19 testing, the level remains low. Nevertheless, the WHO said missed COVID-19 cases are largely because they are asymptomatic and that there is no evidence of miscalculation of death figures.

"Africa has not witnessed an exponential spread of COVID-19 as many initially feared," Moeti said. "But the slower spread of infection in the region means we expect the pandemic to continue to smolder for some time, with occasional flare-ups."