Days after the mass shooting at a gay nightclub in Orlando, House members and the White House made seemingly contradictory claims concerning the Federal Drug Administration’s blood donation policy for gay and bisexual men.

California Rep. Barbara Lee said on June 14, for example, that the FDA’s one-year deferral policy, or celibacy, for men who have sex with men (MSM) is “based in fear and stigma, not science.” The White House, on the other hand, claimed on the same day that the FDA’s current policy does “rely on scientific advice.”

In fact, the FDA’s policy is supported by scientific evidence. However, Lee and others’ proposed alternative – an individual risk-based assessment which would evaluate whether potential donors’ behaviors put them at risk for transfusion-transmissible infections – also has scientific evidence to back it up.

On Dec. 21, 2015, the FDA switched its original 1985 MSM blood donation policy from a ban on any man who has had sex with another man since 1977 to a one-year deferral. The revised policy also defers blood donations from a woman who has had sex with an MSM for one year.

The FDA cites the high prevalence of human immunodeficiency virus, or HIV, in the MSM population as support for its one-year deferral policy. While all blood donations are tested for HIV, current tests don’t work until approximately nine days after the virus is first transmitted, a so-called “window period.” Blood donations from MSM during this time pose a risk to the blood supply, the FDA says.

The FDA also uses evidence from other countries to support its case. For example, between 1996 and 2000, Australia changed its policy to a one-year deferral for MSM and saw no increase in HIV transmissions via blood transfusions. Before this change, Australian state and territory policies ranged from a lifetime ban to a five-year deferral for MSM.

But the House members and other critics of the FDA policy have argued that a one-year deferral for MSM is unnecessary because advances in HIV testing have substantially reduced the risk of transmitting the virus via blood transfusions. They also cite the FDA’s own revised policy document, which states that HIV prevalence in MSM blood donors is much lower than HIV prevalence in the MSM population as a whole.

Still, the FDA argues there is “inadequate data” to support an individual risk-based assessment for MSM blood donations for the U.S. population.

How much scientific evidence is enough for policy change is matter of opinion.

We take no position on whether the FDA’s blood donation policy is discriminatory against gay and bisexual men. But we can outline the scientific evidence backing each side of this ongoing debate.

The FDA’s Case

On June 12, 49 people were killed and 53 wounded at Pulse, a gay nightclub in Orlando, Florida, when Omar Mateen, a 29-year-old U.S. citizen launched an attack in the name of Islamic State terrorist group. After the shooting, blood banks in the region advertised a need for donors, according to the New York Times. But many gay and bisexual men, the population most affected by the shooting, weren’t allowed to donate because of the FDA’s current one-year deferral policy for MSM.

On June 14, White House Press Secretary Josh Earnest told a reporter that the FDA’s decision to change its original 1985 recommendation on MSM blood donations to a one-year deferral was “a policy change that was made consistent with the advice of our best scientists and public health professionals.” He added that “the president believes that when it comes to these kinds of questions, that we’re going to rely on scientific advice.”

According to the Centers for Disease Control and Prevention, the term “MSM” indicates a behavior, not a sexual preference. In other words, as the United Nations explains, the term includes sexual acts between men who don’t identify as gay or bisexual. The CDC and AIDS.gov also note that unprotected anal sex is the riskiest type of sex for HIV transmission, in part, because of the biology of the rectum.

Today, MSM are the population most affected by HIV. MSM are roughly 2 percent of the U.S. population, according to the CDC. Yet, in 2012, 56 percent of people living with HIV were MSM. In 2010, 63 percent of new HIV infections in adults were attributed to male-to-male sexual contact. This indicates that the MSM population remains at “high risk of HIV exposure,” writes the FDA in its Dec. 21, 2015 revised policy document.

According to a study cited by the FDA and sponsored by the National Heart, Lung and Blood Institute, a history of male-to-male sexual contact was associated with a 62-fold increase in risk of HIV transmission among blood donors compared to a control group. In contrast, a history of multiple sexual partners of the opposite sex in the last year increased the risk of HIV transmission among blood donors by 2.3-fold.

Other countries with similar HIV statistics and blood donation screening methods to the U.S. have also implemented one-year deferral policies for MSM and experienced no increased HIV risk to its blood supply, notes the FDA.

Australia, for example, saw no change in risk to the blood supply after it implemented a one-year deferral policy. Risk for this study was “defined by the number of HIV positive donations per year and the proportion of HIV-positive donors with male-to-male sex as a risk factor,” writes the FDA.

The FDA argues that sufficient data aren’t available to assess whether MSM blood donation deferral policies of less than one year would pose a risk to the blood supply.

The FDA also says “individual risk-based options were not determined to be viable options for a policy change at this time for a number of reasons.” For example, it says “individual risk assessment by trained medical professionals would be very difficult to validate and implement in our current blood donor system due to resource constraints.”

At present, blood donation assessments aren’t necessarily carried out by trained medical professionals in the U.S., Louis Katz, a trained doctor and the chief medical officer at America’s Blood Centers, told us over the phone. In contrast, in other countries, such as Italy, these assessments by law must be conducted by specifically trained professionals.

The FDA also argues an individual risk-based approach isn’t viable because “the available epidemiologic data … do not support the concept that MSM who report mutual monogamy with a partner or who report routine use of safe sex practices are at low risk for HIV.” Specifically, FDA says “the rate of partner infidelity in ostensibly monogamous heterosexual couples and same-sex male couples is estimated to be about 25%.” The FDA also cites a condom failure rate of roughly 1 to 2 percent per episode of anal sex.

Conversely, many proponents of the individual risk assessment for MSM blood donations argue MSM in monogamous relationships are at low risk for HIV transmission.

Based on the papers the FDA cites, we weren’t able to independently verify infidelity rates in monogamous same-sex couples. We reached out to the FDA for clarification, but have yet to hear back. If we do receive a response, we will update this article accordingly.

Steven Kleinman, senior medical advisor for AABB, formerly known as the American Association of Blood Banks, told us by email that the organization supports the FDA’s 12-month deferral for MSM. He said “science supports that it would not increase risk of HIV transmission through transfusion.”

Kleinman also said the policy is “in line with the deferral period for other HIV risk factors,” such as a person who has had sex with an HIV-positive individual, an intravenous drug user or a prostitute. For these categories, the FDA also defers blood donation for 12 months.

A representative for the American Red Cross provided us with similar comment by email, saying it supports the FDA’s one-year deferral policy because it’s in line with deferrals for the other risk categories noted above. The representative added that, “At present, there are insufficient scientific data available to determine whether it’s safe to rely only on individual behavioral risk factors when determining donation eligibility. The FDA selected the one-year deferral to provide adequate time for the detection of all infected individuals.”

Katz, at America’s Blood Centers, agreed with the Red Cross. He told us in a phone interview that there are insufficient data to say risk assessment on an individual basis for MSM wouldn’t pose a risk to the blood supply. Katz also emphasized the complexity of the MSM blood donation issue. Given this complexity, the FDA’s “evidence threshold is incredibly high,” he said.

Overall, the FDA is concerned with HIV transmission from MSM donors during the so-called “window period,” when current tests often can’t detect the virus in blood. To support its case, the FDA cites the fact that MSM population is disproportionately affected by HIV. It also uses data collected in other countries, like Australia, which showed no increased risk to its blood supply after the implementation of a one-year deferral policy for MSM.

The Congress Members’ Case

On June 14, Illinois Rep. Mike Quigley put out a press release that included comment from Rep. Barbara Lee and other House Democrats on the FDA’s blood donation policy for MSM.

Like Lee, New York Rep. Sean Patrick Maloney called the FDA’s policy a “bigoted, backward and unscientific regulation.” California Rep. Xavier Becerra also implied science wasn’t the basis for the FDA’s policy when he said, “Science should be the basis for our policy, not sexual orientation.”

Quigley, the vice chair of the Congressional LGBT Equality Caucus, also called on FDA Commissioner Robert Califf “to change the policy to be based on the risk of transfusion-transmissible infections, and not on sexual orientation.” In other words, an individual risk-based assessment.

In addition, Florida Rep. Alcee L. Hastings said, “We have the technological capabilities to screen blood donations to ensure they are safe for use, regardless of one’s sexual orientation.”

When we contacted Lee’s office for comment, a representative told us by email that, “The scientific evidence is clear. MSM blood donors are, in fact, LESS LIKELY to have HIV than the overall population.”

This is true. According to the FDA’s own revised blood donation policy document, “The prevalence of HIV infection in male blood donors who reported that they were MSM was determined to be 0.25%, which is much lower than the estimated 11-12% HIV prevalence in the population of individuals reporting regular MSM behavior … This indicates that considerable self-selection likely took place in individuals who presented to donate.”

A representative from Maloney’s office also pointed us to a commentary paper published in the Columbia Medical Review in June 2015, before the FDA changed its policy to a one-year deferral in December 2015. However, the authors — R.T. Winston Berkman and Li Zhou, law and public administration graduate students at New York University and Columbia University, respectively — do address the FDA’s then proposed one-year deferral policy for MSM.

The paper’s authors note that “significant advancements have been made in HIV testing” since the 1980s. In fact, these advancements in HIV donor testing and other factors have reduced the risk of HIV transmission via blood transfusion from about 1 in 2,500 to 1 in 1.47 million, according to the FDA’s revised policy document.

For comparison, the odds of being struck by lightning in a lifetime (80 years) is 1 in 12,000, according to the National Oceanic and Atmospheric Administration. The odds of being struck by lightning in a given year is 1 in 960,000.

Berkman and Zhou also claim that all donated blood is tested for HIV prior to use. This is also true. A representative from the American Red Cross told use by email that, “Every unit of blood is screened for infectious diseases including HIV.”

However, while “HIV tests currently in use are highly accurate,” said the Red Cross, they “still cannot detect HIV 100 percent of the time.” The 1 in 1.47 million risk of HIV transmission via blood transfusion is “due almost exclusively from so-called ‘window-period’ donations,” the Red Cross told us.

During the window period, “a person could test negative, even when they are actually HIV positive and infectious. Therefore, blood donors are not only tested but are also asked questions about behaviors that increase their risk of HIV infection,” the Red Cross said.

So while “technological capabilities” greatly minimize the risk of HIV transmission via blood transfusion, they can’t “ensure” donations are safe for use, as Hastings claimed.

Still, Katz told us in a phone interview that he’s “certain a year is too long” because the “window period” for HIV and other pathogens don’t come close to a year. “The same can be said for six months,” he added. Still, he reiterated that the data required by the FDA to support the safety of implementing shorter deferral periods or an individual risk-based assessment for the U.S. population is not yet available.

Berkman and Zhou also cite evidence from Italy to make their case. In 2001, Italy changed its blood donation for MSM from a lifetime ban to an individual risk-based assessment.

In a paper published in in Blood Transfusion in July 2013, Barbara Suligoi, an epidemiologist at the Istituto Superiore di Sanita in Rome, and others saw “no significant increase in the proportion of MSM compared to heterosexuals … among HIV antibody-positive blood donors” after the new policy was implemented in Italy. Based on their results, the researchers argue the policy change didn’t have “a significant impact” on the HIV epidemic in the country.

But the authors also write, “our results may not be generalised to countries with different HIV epidemics, different awareness of sexual behaviours at risk, or different procedures in blood donor selection.”

In fact, in Italy in 2011, MSM accounted for 33.2 percent of new HIV infections, whereas heterosexuals accounted for 45.6 percent. Conversely, MSM accounted for the majority of new HIV infections in the U.S. in 2012, as previously noted. In Italy blood donation risk assessments are also carried out by specially trained medical professionals — which is not necessarily the case in the U.S.

Thus, proponents of the individual risk-based assessment are comparing apples to oranges when they cite Italy as evidence for their proposed policy’s safety. In fact, the FDA argues, “Although individual donor assessment for risk of HIV and other infections has been implemented in a few countries, significant differences exist regarding the situation in those countries and the situation in the United States.”

Also, based on the Columbia Medical Review paper, Maloney’s representative told us by email that, “Gay and bisexual men who have lived in monogamous relationships and practice safe sex are not at significant risk for transferring HIV or other blood-borne illnesses.” A representative from Lee’s office also made a similar claim in a phone interview.

In theory, this is true. But as we’ve noted, the FDA argues the “available epidemiologic data … do not support the concept that MSM who report mutual monogamy with a partner or who report routine use of safe sex practices are at low risk for HIV.” However, we weren’t able to verify this claim based on the evidence the FDA provided.

The congressional members and other proponents of the individual risk-based assessment for blood donations from MSM have made some worthy points. For one, MSM blood donors are much less likely to have HIV than the overall population, which suggests many HIV-positive MSM are already abstaining from donating. Proponents also correctly claim that significant advancements in HIV testing have substantially reduced the risk of transmitting the virus via blood transfusion.

However, we reiterate that how much or what scientific evidence is needed for policy change is a matter of opinion.

Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.