Job postings requiring a Covid-19 vaccine have jumped sharply in the past month among a wide range of businesses, according to a new analysis published Thursday by job site Indeed.

The increase comes as the highly contagious delta variant has fueled a surge in virus cases, vaccines have become more broadly available and more businesses mandate vaccination as they bring their workforces back to the office.

The share of job posts that require a Covid vaccine were up 34% on Aug. 7 when compared to the prior month, according to Indeed data. (The analysis uses a seven-day moving average.)

Other jobs ads aren’t as specific they ask for vaccination without explicitly mentioning Covid. Those listings are up 90% over the same period.

“They don’t mean the polio vaccine. It’s so apparent [they mean Covid-19],” according to AnnElizabeth Konkel, an economist at the Indeed Hiring Lab who authored the analysis.

The jump in businesses requiring a vaccine for new hires also coincides with a record number of U.S. job openings in June.

Despite big percentage growth, the overall number of job posts mandating applicants to have a vaccine is still relatively small, Konkel said.

In the software-development sector, for example, about 438 job postings out of every 1 million required a vaccine in July. But that represents an increase of more than 10,000% from the 3.5 posts per million that did so in February, according to Indeed. To be sure, Covid vaccines weren’t as widely available earlier this year.

The trend is similar in many other industries and occupations that hadn’t mandated vaccination in great numbers, like accounting, retail and marketing, according to the analysis.

“Broadly, the trend is more job postings are requiring the Covid-19 vaccine,” Konkel said. “I really think we’re on the cusp of a potentially booming trend.”

The seven-day average of new Covid cases on Wednesday (more than 114,000 infections) was up 64% from just two weeks earlier, according to Centers for Disease Control and Prevention data. The average number of cases is at its highest level since February.