Google on Wednesday slowed its plan to get campuses back up and running, and said workers returning to offices will need to be vaccinated against Covid-19.

Spikes in infections due to a Delta variant of the virus have ramped up concerns about returning to offices, prompting Google to extend its global work-from-home option through October 18, according to chief executive Sundar Pichai.

"Anyone coming to work on our campuses will need to be vaccinated," Pichai said in a blog post.

"We're rolling this policy out in the United States in the coming weeks and will expand to other regions in the coming months."

Implementation of the policy will be adapted to local conditions, including vaccine availability, according to the Silicon Valley based tech titan.

"I hope these steps will give everyone greater peace of mind as offices reopen," Pichai said.

"Seeing Googlers together in the offices these past few weeks filled me with optimism, and I'm looking forward to brighter days ahead."

Google was among the tech companies that abandoned campuses early last year, letting people work remotely rather than risk exposure to Covid-19 in offices.

Google has been paying the salaries of campus workers unable to do their jobs because of closed offices, and helping employees get access to vaccines, according to Pichai

"Even as the virus continues to surge in many parts of the world, it's encouraging to see very high vaccination rates for our Google community in areas where vaccines are widely available," Pichai said.

"This is a big reason why we felt comfortable opening some of our offices to employees who wanted to return early."