By Anna Hirtenstein

The coronavirus pandemic has led to the genesis of a new class of bonds, paving the way for fund managers to springboard from socially responsible investing to putting money toward the health crisis.

Governments, banks, companies and multinational organizations raised $151.5 billion globally by May 31 from selling Covid-19 bonds, or debt whose proceeds are broadly earmarked for work linked to the pandemic, according to research by BNP Paribas.

Pharmaceutical giant Pfizer Inc., Bank of America Corp. and Bank of China are among those that have tapped investors with such bonds since February, though sometimes even the issuers don't classify the debt as virus bonds.

There aren't set rules on what constitutes a Covid-19 bond, making it the latest example of investments with broad claims and murky definitions that have drawn large pools of money in recent years. Some part of the proceeds from the virus bonds are being used to fund the development of vaccines or treatments, or to bolster health-care systems and curtail the outbreak. In other cases, the funds may go toward relief efforts, but there is no system in place to track their ultimate use.

The Covid-19 bonds are generating more interest than conventional ones for now, said Agnès Gourc, co-head of sustainable finance at BNP Paribas. The French bank has been the bookrunner for 12 coronavirus bond sales so far.

The debt has proven particularly attractive to investors who are interested in socially responsible investing, and who value environmental, social and governance practices. The growth of ESG investing in recent years helped the creation of virus-related bonds when the pandemic broke, allowing markets to quickly redirect the flow of capital to the health crisis, according to investors and bankers.

"These things don't happen overnight," said Yo Takatsuki, head of ESG research and active ownership at AXA Investment Managers. "Covid-19 bonds are a very natural progression and development from a lot of the structures we've been building in the market over the past decade" for the sustainable-finance industry, he said.

The Covid-19 bonds have attracted more money this year than bonds linked to sustainability projects, a corner of the debt market that has received a lot of attention due to growing interest in climate-change issues. Sustainability bonds, a broad term encompassing securities that raise capital for environmentally-friendly or for social-impact projects, drew $116.8 billion from investors by the end of May, according to BNP Paribas.

The virus bonds should be a financing tool to mitigate the adverse impacts of the pandemic and to address the recovery efforts, said AXA. The insurer's asset-management division is asking issuers to be transparent about how the money will be used, and said it isn't taking the label at face value. AXA has invested approximately EUR350 million ($391 million) in coronavirus bonds to date, including those issued by the World Bank and the European Investment Bank.

In common with so-called green bonds, whose proceeds are earmarked for environmental projects, and other ESG-related debt, the characteristics of a coronavirus bond can vary depending on the type of issuer or the country. Virus bonds issued in China only have to use 10% of the proceeds toward relief efforts, while development banks tend to dedicate all the money, according to BNP Paribas.

Pfizer raised $1.25 billion by selling 10-year debt in March labeled as sustainability bonds. That debt falls into the category of Covid-19 bonds, according to BNP Paribas, as the funds are to be used for work to address the pandemic, among other things. But Pfizer disagrees with that categorization, underscoring the lack of consensus on what defines a coronavirus bond. The drugmaker said it began exploring the potential for a sustainability bond before the pandemic.

Bank of America said the $1 billion it raised from what it labeled a corporate social bond last month would be used to "support the fight against the Covid-19 pandemic" by lending to the health-care industry. The lender said the bond was the first of its kind from a U.S. commercial bank, and the seventh ESG-themed bond since 2013 from Bank of America.

In China, more than 200 companies, including manufacturers, airlines and property developers, collectively raised over $74 billion by May 26 from selling more than 700 coronavirus bonds, according to data provider Wind Info. Despite their label, some of the bonds' proceeds are being used to refinance companies' debt, which is helping them avoid defaults.

Broad demand for such bonds could in theory result in lower costs for the borrowers, but the market is too nascent for a definite conclusion, Ms. Gourc said. The coupon on the Pfizer bonds was 2.625%, comparing with a coupon of 3.450% for conventional bonds of the same maturity issued in 2019.

More such bonds are likely to come to market, according to Ms. Gourc, as investors are increasingly interested in allocating their funds to social-impact investing.

"If you have the same issuer having a conventional or Covid bond, do you have a preference?" she said. "All investors will have a preference for the ESG-type bond."

Write to Anna Hirtenstein at anna.hirtenstein@wsj.com