The global response to the HIV pandemic since the 2000s has been an unprecedented success, with rapid scale-up of HIV prevention and antiretroviral treatment (ART) programs [1]. Although most low-and-middle-income countries are transitioning towards domestic public funding because of shrinking donor funding, Official Development Assistance (ODA) remains an essential component of HIV financing, especially in sub-Saharan Africa [2]. The coronavirus disease 2019 (COVID-19) pandemic produced an economic crisis characterized as the worst since the Great Depression in the 1930s [3]. The International Monetary Fund (IMF) estimates that reduced economic output and trade because of COVID-19 -- and its impactful countermeasures, such as lockdowns and curfews -- will shrink the world gross domestic product (GDP) by 4.4% in 2020, and will only gradually recover over the next years [4]. These declines can have devastating effects on donor and domestic healthcare financing for the years to come, including for HIV. We estimated the potential impact of the global recession on HIV financing in the 10 countries with the largest ART programs in sub-Saharan Africa -- Ethiopia, Malawi, Mozambique, Tanzania, Uganda, Kenya, Nigeria, Zambia, Zimbabwe, and South Africa -- over the next 5 years to illustrate potential gaps due to the economic crises, and propose urgent mitigation strategies to ensure continued service delivery. Together, these countries account for roughly 70% of HIV-infected people worldwide [5] and spend an estimated US$7.5 billion on HIV annually, of which almost two-thirds (i.e. about US$ 4.7 billion) comes from ODA [6]. We used IMF economic projections on GDP growth and published estimates of current domestic spending and ODA for HIV in each country [4,5]. We first calculated the share of GDP spend on HIV from domestic sources, and the proportional share of ODA from donor countries. We then developed four scenarios to reflect possible effects of GDP declines and political choices in HIV financing, and calculated the net difference to a no-COVID-19 counterfactual using IMF country-specific economic projections from before the pandemic (Fig. 1).Fig. 1: Estimated impact of the coronavirus disease 2019-related global recession on HIV financing in the 10 countries with the largest HIV programs in sub-Saharan Africa.Figure 1 shows the projected net difference in country-specific HIV/AIDS budget over the next 5 years for four scenarios, compared with a no-COVID-19 counterfactual, for Ethiopia, Malawi, Mozambique, Tanzania, and Uganda (low-income), Kenya, Nigeria, Zambia, and Zimbabwe (lower-middle-income), and South Africa (upper-middle-income). Estimates were generated using data on projected country-specific GDP growth from the IMF [4], and latest data on country-specific domestic and donor funding for HIV/AIDS as provided by the Joint United Nations Programme on HIV/AIDS (UNAIDS) [5]. All estimates are reported in 2019 US$. If the share of GDP designated for HIV in both domestic and donor countries remains unchanged, we estimate a gap of about US$1.9 billion over 2020–2024, a decline of about 5% compared with funding over 2015–2019 (Fig. 1, scenario 1). If the share of GDP designated for HIV falls by an initial 10% in both domestic and external sources in 2020 and 5% in the subsequent years -- representing a shift of funding to other sectors, such as COVID-19-related healthcare, economic relief funds, or payment of government debts -- we estimate a cumulative gap of US$4.1 billion, amounting to a decline of about 11% in total funding for HIV (Fig. 1, scenario 2). Compensating for gaps in ODA would require substantial increases in domestic HIV funding: by roughly 10% (upper middle-income countries), 15% (lower middle-income countries) or 20% (lower income countries) (Fig. 1, scenario 3). Even in an extremely optimistic scenario, in which the absolute amount of donor funding remains unchanged, domestic HIV spending would need to increase by about 5% to maintain current funding levels (Fig. 1, scenario 4). Such increases appear unlikely, as most of these countries already struggled to expand domestic HIV funding in the pre-COVID-19 era [6]. Although it is challenging to speculate which of these scenarios will predominate, current events suggest that our most pessimistic scenario, including additional cuts in donor funding on top of the budget declines caused by GDP fluctuations (scenario 2), is at least partly unfolding. The UK government recently announced to consider temporarily cutting foreign aid spending from 0.7% to 0.5% of gross national income [7], a decline of over 25%, and other donor countries may follow suit. For nine out of the 10 countries studied, 77--94% of funding for care and treatment comes from ODA [6]; making these political decisions a direct threat to continuation of ART programs. The only exception is South Africa, where only 18% of the HIV budget stems from ODA and continuation largely depends on local political decisions. However, as South Africa has been severely affected by the pandemic, reprioritization of domestic resources may well be necessary to finance the COVID-19 response and economic recovery. If we assume scenario 2 to unfold, we can roughly calculate the detrimental effects on service provision for HIV-infected people. For the 10 countries in our analysis, about US$3 billion [6], enabled 13.6 million people to receive life-saving ART pre-COVID-19 [5]. If relative funding distributions between treatment and prevention are maintained, the most extreme funding shortfalls of $850 million for 2021 (11% compared with 2019) in scenario 2 could, therefore, mean that approximately 1.5 million people enrolled in HIV care could not be served any longer. Interruption of ART could have direct implications for the health of HIV-infected people: potentially resulting in increased incidence of AIDS-defining illnesses, treatment resistance, and AIDS-related mortality. Moreover, extended lockdowns could lead to decreased ART adherence because of poor access and drug stock-outs, and many key-population outreach programs have been reduced or halted. Several mathematical modelers forecast that, of all interventions currently in place for HIV prevention and control, interruption of ART delivery because of COVID-19 will have the biggest impact on relative changes in mortality, and, together with interrupted condom availability, the biggest impact on relative changes in incidence [8]. Whereas shifting funding within limited HIV budgets away from prevention programs to ensure treatment continuation during the crisis might seem to be an attractive short-term solution, this could have a detrimental impact on the HIV epidemic in the long-run. The complex nature of HIV financing makes creating accurate predictions challenging, and our estimates should thus be interpreted as cautionary rather than predictive. In our analyses, we explored the effects of the COVID-19-related recession on HIV financing by making a simplified assumption of a linear relationship between country-specific GDP and HIV financing. Although this seems reasonable, validating our assumption against past observations (e.g., trends in GDP versus trends in HIV financing) is complex, as detailed yearly country-level data is not always available. Next to the direct GDP-dependent effects of COVID-19 countermeasures, several political decisions and indirect economic disruptions might further adversely affect HIV financing. First, aid distribution for HIV might increasingly be prioritized for low-income countries alone. Although this might mean that the poorest countries in the world are slightly more protected, lower middle-income and upper middle-income countries would need an accelerated process towards country-ownership. Second, several non-governmental financing schemes will be disproportionally affected by the pandemic. For instance, the air tax levy used to finance Unitaid, accounting for approximately 6% of the Global Fund to Fight AIDS, Tuberculosis and Malaria budget [9], will be severely affected by air travel restrictions. Third, financial gaps and shifts in sources indirectly supporting the HIV response, such as international research funding or the announcement by the USA to halt funding to the WHO [10], might adversely affect the HIV response and its financing for years to come. Fourth, domestic funding for HIV may be affected by the severity of the COVID-19 pandemic in the African nations and the political response. The proportion of the population infected with COVID-19, the strength and resilience of the local health system to respond to the COVID-19 pandemic, and the political structure and leadership in a country are all factors that are likely to influence the level of funding appropriated to fight HIV. Regardless of how the financial and political complexities unfold and affect prevention and treatment programs in the individual countries, it is critical to implement mitigation strategies to compensate for the financial gaps. First, continuation of HIV service delivery could be ensured and efficiency improved through accelerated implementation of differentiated care -- by enabling less frequent ART pick-ups [11], shifting tasks to community health workers [12], integrating HIV/COVID-19 prevention [13], and focusing on prevention for key populations. Second, resources could be pooled at regional level (facilitated for instance through the Southern African Development Community) to better leverage emergency funding, sharing of human capital and knowledge, and expertise in health program management. Third, integration with other health services could improve efficiency, and simultaneously accelerate progress towards universal health coverage [14,15]. Integration will also force a conversation about reprioritization across all health areas, essential to overcome the broader health system challenges introduced through the COVID-19 pandemic. Most importantly, mitigation strategies should strengthen the HIV response by using existing resources more efficiently, rather than shifting funding from other resources to fill gaps, so that health service provision for other population needs is not endangered. The COVID-19 pandemic is putting acute pressure on HIV programs in high-burden countries. With potential declines in funding of as much as 5% or 11%, the scale-up of HIV prevention and treatment in sub-Saharan Africa could come to halt or even reverse. But, as with any major crisis, the pressure to innovate under challenging circumstances could ultimately result in an HIV response and general health system that is more integrated, efficient and sustainable, improving resilience to future crises, and bolstering efforts to end the HIV pandemic by 2030. Acknowledgements Conflicts of interest There are no conflicts of interest.