COVID-19 vaccines under development are not guaranteed to work and people who say to expect a vaccine before year-end are doing a “grave disservice to the public,”Merck & Co Inc’s chief said, according to a Harvard Business Review report.

The potential vaccines may not have the qualities needed to be rapidly deployed in large numbers of people, Chief Executive Kenneth Frazier said in an interview published on Monday.

“If you’re going to use a vaccine on billions of people, you better know what that vaccine does.” A US official said Monday that drug-makers partnered with the US government are on track to begin actively manufacturing a COVID-19 vaccine by the end of the summer, Reuters reported.

The Trump administration aims to produce 300 million vaccine doses by the end of 2021 though its Operation Warp Speed Programme.

Some previous vaccines “not only didn’t confer protection,but actually helped the virus invade the cell, because it was incomplete in terms of its immunogenic properties,” Frazier said.

“So, we have to be very careful.” Merck announced in May plans to study potential vaccine and therapy candidates for COVID-19 through partnerships and an acquisition of Austrian vaccine maker Themis Bioscience. It has not started clinical trials for its vaccine.

Frazier, one of only four Black CEOs of Fortune 500 companies, said the US pandemic, with a higher death rate among non-white people, has highlighted “huge structural elements of racism that have existed in this country for a long time.”

American companies must work to dismantle processes and systems that impede Black employees from advancing, he said. “At the end of the day, if you’re complacent with the status quo,you’re complicit in the racism that the status quo hides.”