SEATTLE—Amazon has pushed back its return-to-office date for tech and corporate workers until January as COVID-19 cases surge nationally due to the more contagious delta variant. Unlike its Seattle-area rival Microsoft and other tech giants, Amazon will not mandate employees receive the COVID-19 vaccine before they return to the office. Instead, the company said Thursday that unvaccinated employees will be required to wear masks in the office. The surge of the Delta variant of the coronavirus has upended many companies’ plans to bring office workers back this fall, a drive already complicated by efforts to accommodate widespread employee preference for flexible remote work policies, and debates over how to handle vaccine and masking policies. Other companies that have postponed reopening plans include Microsoft, Google, Twitter, and Lyft. Amazon, which had previously set a Sept. 7 return date, said employees will now be expected to report to its U.S. offices on …