The lower risk of dementia that seems to be associated with vaccination against shingles might not have anything to do with protection from the virus.

It might be the adjuvant in the vaccine itself. 

Also, ivermectin might be useful in controlling malaria. But its effects are on the mosquitoes, not humans. 

References

• Lower risk of dementia with AS01-adjuvanted vaccination against shingles and respiratory syncytial virus infections 
• Ivermectin to Control Malaria — A Cluster-Randomized Trial | New England Journal of Medicine
• Ivermectin against Malaria — Good News in Bad Times | New England Journal of Medicine