Priority age targets for COVID-19 vaccination in Ethiopia under limited vaccine supply

Background The worldwide inequitable access to vaccination claims for a re-assessment of policies that could minimize the COVID-19 burden in low-income countries. An illustrative example is what occurred in Ethiopia, where nine months after the launch of the national vaccination program in March 2021, only 3% of the population received two doses of COVID-19 vaccine. In the meantime, a new wave of cases caused by the emergence of Delta variant of SARS-CoV-2 was observed between July and November 2021. Methods We used a SARS-CoV-2 transmission model to estimate the level of immunity accrued before the launch of vaccination in the Southwest Shewa Zone (SWSZ) and to evaluate the impact of alternative age priority vaccination targets in a context of limited vaccine supply. The model was informed with available epidemiological evidence and detailed contact data collected across different socio-demographic settings. Results We found that, during the first year of the pandemic, 46.1-58.7% of SARS-CoV-2 infections and 24.9-48% of critical cases occurred in SWSZ were likely associated with infectors under 30 years of age. During the Delta wave, the contribution of this age group in causing critical cases was estimated to increase to 66.7-70.6%. However, our findings suggest that, when considering the vaccine product available at the time (ChAdOx1 nCoV-19; 65% efficacy against infection after 2 doses), prioritizing the elderly for vaccination remained the best strategy to minimize the disease burden caused by Delta, irrespectively to the number of available doses. Vaccination of all individuals aged 50□years or older would have averted 40 (95%CI: 18-60), 90 (95%CI: 61-111), and 62 (95%CI: 21-108) critical cases per 100,000 residents in urban, rural, and remote areas, respectively. Vaccination of all individuals aged 30□years or more would have averted an average of 86-152 critical cases per 100,000 individuals, depending on the setting considered. Conclusions Despite infections among children and young adults likely caused 70% of critical cases during the Delta wave in SWSZ, most vulnerable ages should remain a key priority target for vaccination against COVID-19.