Canadian Imperial Bank of Commerce and National Bank of Canada said Wednesday they have asked staff in Canada to work remotely, joining Bank of Nova Scotia in halting plans for a return to offices amid growing concerns over the Omicron variant.

Canada's top health official Theresa Tam warned on Monday that COVID-19 cases in the country could rise rapidly in the coming days, leading banks and financial firms to rethink return-to-office plans.

"We've made the decision to pause our plans" for a return to work locations in January, Sandy Sharman, group head for people, culture and brand at CIBC, the country's fifth largest lender, said in an internal memo.

Sharman cited increases in COVID-19 cases and uncertainties due to the new variant. Staff who had recently returned to on-site work were also being asked to work remotely. "It's our intent to return as soon as it's safe to do so."

Earlier on Wednesday, National Bank, the country's sixth-biggest lender, confirmed a similar message to employees.

"A message was just sent to our employees asking them to work remotely if they can," a spokesperson told Reuters in an email. "In the longer term, we are staying the course with our plan of reopening gradually, on a voluntary basis, with no set date."

The rapid spread of the Omicron strain has wreaked havoc on companies' efforts to return to normalcy.

Bank of Nova Scotia, Canada's third largest lender, said Monday it would pause its plan for employees working remotely to return to its Toronto head office starting on Jan. 17.

In the United States, JPMorgan said Wednesday it was making its annual healthcare conference in San Francisco a "virtual" event. The decision came after major biotech firms like Moderna and Amgen said they would not attend in person.

New York and New Jersey are experiencing the fastest spread of Omicron in the United States, according to data from the U.S. Centers for Disease Control and Prevention.

A source at one major U.S. bank headquartered in Manhattan said there was "heightened diligence" as a result of the Omicron variant. A number of staff were choosing to work from home out of caution ahead of the holidays, the source said.

Insurer Prudential has pushed back its return to office plan in the United States to January on a voluntary basis, a source with knowledge of the matter told Reuters on Wednesday. However, that decision was made prior to Thanksgiving, the source said.

Canadian insurer Sun Life Financial said on Tuesday it had paused its office re-opening pilot for more employees until the end of January.

Last week, Jefferies asked staff to work from home for the remainder of the year, due to a spate of COVID-19 cases. On Tuesday, JPMorgan told unvaccinated Manhattan staff to work from home.

Morgan Stanley Chief Executive James Gorman said Monday he expects COVID-19 to be an issue through the next year.

(Reporting by Noor Zainab Hussain and Manya Saini in Bengaluru, and Nichola Saminather in Toronto; Writing by Matt Scuffham; Editing by Anil D'Silva, Mark Heinrich and Richard Pullin)