Kentucky Gov. Andy Beshear's son has tested positive for COVID-19 as cases in the state skyrocket.

"My son is generally asymptomatic, we believe because he is vaccinated and boosted," Beshear said Monday.

The governor and the rest of his family tested negative on Monday, according to Beshear's office.

Kentucky reported 81,473 cases for the week ending on Sunday -- a record high.

Hospitalizations are also approaching a record high, his office said.

The World Health Organization is sounding the alarm over rising cases of a new omicron sub-variant.

In an updated post to its website on Monday, the WHO said the new sub-variant, called BA.2, is a descendant of omicron, the now-dominant, highly contagious variant of the novel coronavirus. Unlike omicron, BA.2 is currently not considered a "variant of concern." But because it is spreading in many countries, the WHO is asking governments and scientists across the globe to monitor the situation and study the new sub-variant, as many have already been doing.

As of last Friday, BA.2 had already been detected in at least 40 countries, including the United States. It has already spread quickly in Denmark, where early reports indicate it doesn't appear to be deadlier than its parent variant. The sub-variant is now also spreading rapidly in the United Kingdom.

While BA.2 is a descendent of omicron, it has slightly different mutations. Omicron has several sub-variants, as does the previously dominant, highly transmissible delta variant.

-ABC News' Sony Salzman

Pfizer and BioNTech said on Tuesday they have initiated clinical studies to evaluate an omicron-based vaccine for adults.

The trials will evaluate the "safety, tolerability, and immunogenicity" of the omicron-based vaccine for healthy adults between 18 and 55 years old, the companies said. About 1,400 participants will receive one of three combinations of the companies' current vaccine and the omicron-based vaccine.

"While current research and real-world data show that boosters continue to provide a high level of protection against severe disease and hospitalization with Omicron, we recognize the need to be prepared in the event this protection wanes over time and to potentially help address Omicron and new variants in the future,” said Kathrin U. Jansen, Ph.D., senior vice president and head of vaccine research & development at Pfizer, said in a statement.

The companies said 615 participants will receive a dose of the omicron booster after two doses of the current vaccine. Another 600 participants will receive a dose of the omicron booster after three doses of the current vaccine. The final 205 participants, who have not been vaccinated, will receive three doses of the omicron vaccine.

Pfizer and BioNTech said they expected to have initial findings in the first half of 2022. ABC News has also learned Moderna is expected to start similar human trials of an omicron-specific vaccine within the coming days.

-ABC News' Eric Strauss

London police said Tuesday they are investigating parties that allegedly took place at British Prime Minister Boris Johnon's official residence and executive office while England was under strict lockdown because of COVID-19.

In a statement before the London Assembly, Metropolitan Police Commissioner Cressida Dick announced the investigation into "a number of events" at 10 Downing Street.

"The fact that we are now investigating does not, of course, mean that fixed penalty notices will necessarily be issued in every instance and to every person involved," Dick said. "We will not be giving a running commentary on our current investigations."

Johnson is facing growing anger and calls for his resignation over claims he and his staff flouted COVID-19 restrictions imposed by his government. In the latest allegation, ITV News reported that the prime minister attended a surprise birthday party held for him at his office during the first lockdown in June 2020 and later hosted friends in his apartment upstairs that evening. At that time, people in England were barred from meeting more than one individual outside their household.

A spokesperson for Johnson's office called the claim "totally untrue," telling ITV News in a statement that, "in line with the rules at the time, the prime minister hosted a small number of family members outside that evening."

Earlier this month, Johnson acknowledged for the first time that he went to a garden party at 10 Downing Street during the first lockdown in May 2020. While he didn't explicitly admit that he had broken any rules, the prime minister apologized and said he had considered the garden party to be a work event to thank his staff for their efforts during the pandemic.

It was unclear which events London's Metropolitan Police Service is investigating.