Amid a blood shortage ignited by the coronavirus pandemic, over 500 health professionals signed an open letter Thursday calling on the Food and Drug Administration to eliminate “the scientifically outdated ban” on blood donations from gay and bisexual men, a policy that traces its roots to the early days of the AIDS crisis.

"As you well know, we are on the precipice of a critical nationwide shortage of blood products," the signatories wrote. "This severe shortage could be alleviated if antiquated restrictions lacking scientific data were responsibly lifted."

The letter — addressed to the FDA’s Blood Products Advisory Committee and signed by infectious disease specialists, public health professionals, clinicians and others — comes just two weeks after the FDA revised its blood donation guidelines to shorten the required sexual abstinence period for men who have sex with men to three months from 12 months.

“While the FDA’s recent decision to shorten the prohibition window to 3 months is a step in the right direction, it does not go far enough in reversing the unscientific ban,” the letter states.

The signatories called for “a more scientifically rigorous” approach to screening blood donors that better reflects modern HIV-testing technology.

In their letter, they argue that the acute blood shortage caused by the global coronavirus pandemic is the reason the FDA should act now to further reduce the wait period for gay and bisexual men. In March, Surgeon General Jerome Adams said that an estimated 86,000 blood donations had been lost as a result of canceled blood drives, and since then the public health crisis has only grown worse.

The American Red Cross, however, said in a press release shared with The New York Times on Thursday that the blood supply is secure this month because “many” blood donors stepped up in recent weeks.

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A 2014 study by UCLA Law's Williams Institute found that fully repealing the gay and bisexual blood donation restrictions could potentially unlock over a half million blood units per year from men who have sex with men — a total increase in supply of 2 percent to 4 percent.

The letter adds that lifting the “ban would increase the number of convalescent plasma donors, a promising treatment for COVID-19.”

In recent weeks, gay and bisexual men who have recovered from COVID-19 have sought to donate their antibody-rich blood for experimental convalescent plasma treatments and have been rejected, adding urgency to these new calls for reform.

Thursday's letter, distributed by LGBTQ advocacy group GLAAD, stated that because new blood testing technologies can reliably detect HIV about a week after infection, it is “unscientific and based on outdated antibody-based HIV testing algorithms” to require gay men to abstain from sex for three months prior to donating blood.

Modern HIV tests look for antigens, which appear in the blood days after infection. Older tests looked for antibodies, which appear weeks and perhaps even months after infection. Gay and bisexual men represent about 70 percent of all new HIV infections in the U.S., according to the CDC.

The FDA initially banned gay and bisexual men from ever giving blood in 1983, then revised the lifetime ban in 2015 to a year's sexual abstinence.

Other countries, the letter states, have better addressed the issue by devising new protocols that ask people about recent risky behaviors irrespective of sexual orientation, which it described as “a much more scientifically rigorous and nondiscriminatory approach to maintaining a safe blood supply.”

Men who have sex with men and are in monogamous relationships, "those on Pre-exposure Prophylaxis [PrEP], and those who consistently use condoms have a particularly low risk of HIV acquisition,” the letter states.

In a statement shared with NBC News, the FDA said it "remains committed to gathering the scientific data that support donor deferral policies that are non-exclusive while helping to ensure a high level of blood safety.”

“To investigate the scientific validity of such an approach, the FDA is working to commence a pilot study that will enroll about 2000 men who have sex with men and who would be willing to donate blood," the FDA stated. "This study, being conducted at community health centers in key locations across the United States, could generate data that will help the FDA determine if a donor questionnaire based on individual risk assessment would be as effective as time-based deferrals in reducing the risk of HIV.”

The letter's signatories join a growing coalition of doctors, politicians and LGBTQ advocates in calling for the full repeal of blood donation restrictions on gay and bisexual men.

The FDA's updated guidelines came just weeks after a group of 15 Senators sent a letter to the FDA urging an overhaul in the blood donor guidelines on men who have sex with men.

“The COVID-19 pandemic has created a nationwide blood shortage, and we must work to increase our nation’s supply based on science and facts,” Sen. Tammy Baldwin, D-Wis., one of the signatories, told NBC News after the letter's publication. The FDA “needs to change their discriminatory blood donation policies to address this challenge and help save lives.”

Other 12-month deferral periods have also been shortened under the updated FDA guidelines, including for people who have traveled to areas with certain endemic diseases, injected drugs or participated in commercial sex work.

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