Dec 8 (Reuters) - Investment bank Jefferies Financial Group (JEF.N) on Wednesday cancelled all client parties and most travel, asking employees to work from home when possible due to a spate of COVID-19 cases. U.S. banks have been more assertive than other industries in encouraging employees back to the office although those plans have come under renewed scrutiny due to the rapid spread of the Omicron variant. Jefferies" disclosure raised questions about whether other banks will also review return-to-office plans, mask mandates and travel and entertainment policies. Jefferies, with its headquarters in midtown Manhattan, called its staff back to offices in October. The bank has felt the pandemic"s impact acutely, as its Chief Financial Officer Peg Broadbent died due to coronavirus complications in March 2020. "Our priority now is to best protect every one of you and your families," wrote Chief Executive Richard Handler in a memo seen by Reuters. "Effective today, we are cancelling all social events and entertainment until January 3rd." The firm has had more than 40 new COVID-19 cases this month including 10 on Tuesday, the memo said, adding only very few cases required hospitalization. Handler added that Jefferies was re-imposing a mask mandate in all offices, regardless of vaccination status. The investment bank, which has 3,000 employees worldwide, also has offices in Asia and Europe. More than 95% of Jefferies staff are now vaccinated, and all visitors to Jefferies" offices must be fully vaccinated, Handler said in the memo. Most major U.S. banks have had staff working in offices since the summer. Senior bankers such as Goldman Sachs Chief Executive David Solomon and Morgan Stanley Chief Executive James Gorman have spoken of the benefits of in-person interaction, particularly for younger employees. So far, U.S. banks are sticking to their existing COVID-19 policies although sources at the "big six" firms say they are keeping a close watch on developments. read more Goldman Sachs (GS.N), Morgan Stanley (MS.N) and JPMorgan (JPM.N) have had most workers back at offices on a rotational basis since the summer. Others, like Wells Fargo (WFC.N), Citigroup (C.N) and Bank of America (BAC.N), have taken a more flexible stance. Wells Fargo pushed its return-to-office plans back to January, while Citigroup employees in New York, Chicago, Boston, Philadelphia and Washington, D.C. have been working from the office at least two days a week since Sept. 13. Some U.S. banks are also choosing not to host holiday parties. Wells Fargo said Wednesday it had no firm-wide celebrations planned. Team parties are at the discretion of individual managers. Citi will not be hosting holiday parties this year to help prevent the spread of COVID-19, a source familiar with the matter said. In Europe, where Omicron has spread more rapidly, some banks have cancelled events such as JPMorgan"s annual festive carols reception in London and year-end party in Paris. Deutsche Bank (DBKGn.DE) has told its London staff they can hold small gatherings at the team level. Asset manager Schroders (SDR.L) and the City of London Corporation, which runs London"s historic financial district, ask guests to take rapid tests before some festive events. read more Some large U.S. companies are also pushing back their office-return date due to the Omicron variant. read more Jefferies, which the memo said had seen attendance average as high as 60% many days globally in recent weeks, said it will require booster shots by Jan. 31.