Incidence of Plasmodium falciparum malaria infection in 6-month to 45-year-olds on selected areas of Bioko Island, Equatorial Guinea
Authors: Nchama Vicente Urbano Nsue Ndong , Said Ali Hamad , Mtoro Ali , Bidjimi Gertrudis Owono , Owono Marta Alene , Maye Escolastica Raquel Mansogo , Mangue Martin Eka Ondo , Okomo Genaro Nsue Nguema , Pasialo Beltran Ekua Ntutumu , Ondo Dolores Mbang , Lopez Maria-Silvia Angue , Mochomuemue Fortunata Lobede , Obono Mariano Obiang , Besaha Juan Carlos Momo , Chuquiyauri Raul , Jongo Said Abdallah , Kamaka Kassim , Kibondo Ummi Abdul , Athuman Thabit , Falla Carlos Cortez , Eyono Jeremías Nzamio Mba , Smith Jordan Michael , García Guillermo A. , Raso José , Nyakarungu Elizabeth , Mpina Maxmillian , Schindler Tobias , Daubenberger Claudia , Lemiale Laurence , Billingsley Peter F. , Sim B. Kim Lee , Richie Thomas L. , Church L. W. Preston , Olotu Ally , Tanner Marcel , Hoffman Stephen L. , Abdulla Salim 
Published: July 2021

Abstract
Background
Extensive malaria control measures have been implemented on Bioko Island, Equatorial Guinea over the past 16 years, reducing parasite prevalence and malaria-related morbidity and mortality, but without achieving elimination. Malaria vaccines offer hope for reducing the burden to zero. Three phase 1/2 studies have been conducted successfully on Bioko Island to evaluate the safety and efficacy of whole Plasmodium falciparum (Pf) sporozoite (SPZ) malaria vaccines. A large, pivotal trial of the safety and efficacy of the radiation-attenuated Sanaria® PfSPZ Vaccine against P. falciparum is planned for 2022. This study assessed the incidence of malaria at the phase 3 study site and characterized the influence of socio-demographic factors on the burden of malaria to guide trial design.

Methods
A cohort of 240 randomly selected individuals aged 6 months to 45 years from selected areas of North Bioko Province, Bioko Island, was followed for 24 weeks after clearance of parasitaemia. Assessment of clinical presentation consistent with malaria and thick blood smears were performed every 2 weeks. Incidence of first and multiple malaria infections per person-time of follow-up was estimated, compared between age groups, and examined for associated socio-demographic risk factors.

Results
There were 58 malaria infection episodes observed during the follow up period, including 47 first and 11 repeat infections. The incidence of malaria was 0.25 [95% CI (0.19, 0.32)] and of first malaria was 0.23 [95% CI (0.17, 0.30)] per person per 24 weeks (0.22 in 6–59-month-olds, 0.26 in 5–17-year-olds, 0.20 in 18–45-year-olds). Incidence of first malaria with symptoms was 0.13 [95% CI (0.09, 0.19)] per person per 24 weeks (0.16 in 6–59-month-olds, 0.10 in 5–17-year-olds, 0.11 in 18–45-year-olds). Multivariate assessment showed that study area, gender, malaria positivity at screening, and household socioeconomic status independently predicted the observed incidence of malaria.

Conclusion
Despite intensive malaria control efforts on Bioko Island, local transmission remains and is spread evenly throughout age groups. These incidence rates indicate moderate malaria transmission which may be sufficient to support future larger trials of PfSPZ Vaccine. The long-term goal is to conduct mass vaccination programmes to halt transmission and eliminate P. falciparum malaria.