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 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.

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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.

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.

Cases have tripled or quadrupled in hard-hit areas like Arkansas, Louisiana, Alabama and Mississippi in the past month.

New cases, hospitalizations and deaths are overwhelmingly occurring among the unvaccinated. Just 59% of Americans eligible for a Covid-19 vaccine (anyone 12 years and older) are fully vaccinated, according to the CDC. About half of all Americans are fully vaccinated.

More than a dozen large U.S. corporations, including Walmart, Google, Tyson Foods, United Airlines and McDonald’s have recently announced vaccine mandates for some or all of their workers. The Pentagon on Monday said vaccines would be mandatory for service members by mid-September.

“Since the Town Hall, we’ve heard from many of you that you would feel more comfortable returning to the office if you had more certainty your colleagues were vaccinated,” Heidi Capozzi, McDonald’s global chief people officer, said in an internal note obtained by CNBC.

“We are also being asked by state and local governments to require vaccinations for corporate employees because getting more of the population vaccinated reduces our own chances of being infected and contributes to community protection,” Capozzi continued.

Several big firms have also postponed plans to bring employees back to the office. Facebook, Amazon and Lyft, for example, recently delayed return-to-work plans into 2022.

The growing list of businesses mandating vaccination may normalize the requirement and lead other hesitant employers to do the same, Konkel said. That’s especially likely if the Food and Drug Administration grants full approval to one or multiple Covid vaccines.

The FDA has so far only granted emergency-use approval of the Pfizer, Moderna and Johnson & Johnson vaccines.

The agency is expected to soon give full approval to Pfizer. Dr. Anthony Fauci, the U.S. government’s top infectious disease expert, said he’s hopeful that will occur by the end of August.

“If that’s the case, you’re going to see the empowerment of local enterprises, giving mandates that could be colleges, universities, places of business, a whole variety and I strongly support that,” Fauci said.