It is no secret that the US president’s emergency plan for Aids relief, launched by George Bush in 2003, is one of the most successful public health campaigns in history. Known as Pepfar, it now reaches more than 14 million people with lifesaving HIV treatment. In the past 15 years, it has enabled more than 2.2 million children to be born free from HIV and has reached more than 15.2 million boys and men with voluntary medical circumcisions, a procedure that can prevent new HIV infections.

However, Pepfar’s most striking accomplishment is the sharp drop in new HIV infections among adolescent girls and young women in sub-Saharan Africa. In just three years, these infections have declined by between 25% and 40% in nearly two-thirds of districts with the highest HIV prevalence.

Success is manifested in Pepfar’s Dreams partnership, a $385m (£290m) commitment to reduce HIV infections in adolescent girls and young women in 10 sub-Saharan African countries. The programme is based on a growing body of evidence that empowering young women through mentoring, confidence-building, job training, making sure they stay in education, and giving them access to comprehensive health information and services can reduce HIV infections.

The emphasis on evidence-based prevention addresses the historic failure of Pepfar’s abstinence and fidelity-in-marriage programmes. In its first 12 years, the organisation spent $1.4bn on such projects, but, according to a 2016 Stanford Medical School study, these made no impact on changing sexual behaviour or reducing new HIV infections.

Nonetheless, the Trump administration recently came down in favour of “sexual risk avoidance” education. This is nothing more than a rebrand of the abstinence programming that deprives adolescents of education about HIV, sexually transmitted infections and unintended pregnancy. It means they grow up without the knowledge they need and deserve to have safe sex. Marital fidelity programming undermines efforts to end early marriage and often fails to acknowledge relationships among LGBTQ young people, which results in isolation and exclusion.

In addition, shrinking budgets make it harder to provide treatment, care and prevention on a global scale. The Trump administration’s budget request for 2019 proposed more than $1bn in cuts to the global Aids response. Pepfar is down by 17% from 2017 levels ($4.65bn to $3.85bn). Support for the Global Fund to Fight Aids, TB and Malaria is down by 31% ($1.35bn to $925.1m).

Trump has also reinstated the Mexico City policy, and this time it’s attached to Pepfar funds. Known as the “global gag rule”, the policy prohibits any international organisation from receiving certain categories of US funding if it even discusses abortions with patients. First introduced by Ronald Reagan, the rule has been enforced and rescinded by different administrations since 1984. Despite the fact that George W Bush exempted Pepfar from the global gag rule after reinstating it, evidence shows that the policy curtailed community-based outreach services such as condom distribution and HIV testing.

Trump made no such exemptions, and expanded the policy to all global health assistance, an unprecedented move that affects 15 times more funding. One year later, its devastating impacts are apparent. Earlier this year, I was in Mozambique, a country with a high HIV prevalence, and I saw the immediate damage of the policy on Pepfar-funded programmes. Rural communities have been cut off from access to health services, clinics have been forced to lay off nurses and activistas (community outreach workers), and HIV prevention programming for young people has abruptly ended.

For a country like Mozambique that has decriminalised abortion and built on that momentum to address other local public health issues like HIV and Aids, Trump’s global gag rule halts progress in providing care to communities across the country.

Saving lives cannot become a relic of the past. Favouring ideology over science is careless, short-sighted and extravagantly wasteful. Fifteen years of data collection speaks for itself – the key to success is the marriage of scientific research and rights-based health policy. Now, more than ever, these principles and policies must prevail.