BACKGROUND: The effect of primaquine in preventing Plasmodium vivax relapses 
from dormant stages is well established. For Plasmodium ovale, the relapse 
characteristics and the use of primaquine is not as well studied. We set to 
evaluate the relapsing properties of these 2 species, in relation to primaquine 
use among imported malaria cases in a nonendemic setting.
METHODS: We performed a nationwide retrospective study of malaria diagnosed in 
Sweden 1995-2019, by reviewing medical records of 3254 cases. All episodes of P. 
vivax (n = 972) and P. ovale (n = 251) were selected for analysis.
RESULTS: First time relapses were reported in 80/857 (9.3%) P. vivax and 9/220 
(4.1%) P. ovale episodes, respectively (P < .01). Without primaquine, the risk 
for relapse was higher in P. vivax, 20/60 (33.3%), compared to 3/30 (10.0%) in 
P. ovale (hazard ratio [HR] 3.5, 95% confidence interval [CI] 1.0-12.0). In P. 
vivax, patients prescribed primaquine had a reduced risk of relapse compared to 
episodes without relapse preventing treatment, 7.1% vs 33.3% (HR 0.2, 95% CI 
.1-.3). In P. ovale, the effect of primaquine on the risk of relapse did not 
reach statistical significance, with relapses seen in 2.8% of the episodes 
compared to 10.0% in patients not receiving relapse preventing treatment (HR 
0.3, 95% CI .1-1.1).
CONCLUSIONS: The risk of relapse was considerably lower in P. ovale than in P. 
vivax infections indicating different relapsing features between the two 
species. Primaquine was effective in preventing P. vivax relapse. In P. ovale, 
relapse episodes were few, and the supportive evidence for primaquine remains 
limited.