Google is postponing a return to the office for most workers until mid-October and rolling out a policy that will eventually require everyone to be vaccinated once its sites are fully reopened.

The announcement comes as the more contagious Delta variant of coronavirus is driving a dramatic spike in Covid-19 cases and hospital admissions.

Google's announcement was shortly followed by Facebook, which also said it will make vaccines mandatory for US employees who work in offices. Exceptions will be made for medical and other reasons.

In an email sent to Google's more than 130,000 employees worldwide, CEO Sundar Pichai said the company is now aiming to have most of its workforce back to its offices beginning on October 18 instead of its previous target date of September 1.

The decision also affects tens of thousands of contractors who Google intends to continue to pay while access to its campuses remains limited.

"This extension will allow us time to ramp back into work while providing flexibility for those who need it," Mr Pichai said.

And Mr Pichai disclosed that once offices are fully reopened, everyone working there will have to be vaccinated.

The requirement will be first imposed at Google's headquarters in Mountain View, California, and other US offices, before being extended to the more than 40 other countries where Google operates.

"This is the stuff that needs to be done, because otherwise we are endangering workers and their families," said Dr Leana Wen, a public health professor at George Washington University and a former health commissioner for the city of Baltimore.

"It is not fair to parents to be expected to come back to work and sit shoulder-to-shoulder with unvaccinated people who could be carrying a potentially deadly virus."