There is a fascinating history regarding the lack of seasonality in the tropics and subtropics with influenza and the common cold. I use it to illustrate how most medical researchers, like most people in the bio-sciences, are not strong on causality chain reasoning.
Most of these infections are transmitted due to exposure to high densities of infectious airborne particles. Now sneezing and wet coughs are very obvious sources but when enclosed area environment studies are done general room density seems to be far more important than localized short term high densities when it comes to infection probability with time. Being in close proximity to a sneeze or cough seem to be less important that what happens to those particles afterwards. How good the ventilation of the area is. Air conditioning seeming to be a great concentrator of infectious particles.
Now the big difference between the tropics and temperate zone is the strong seasonality in the ventilation of enclosed spaces. Especially homes. Not much change year round in the tropics but not too many people keep windows and doors open in the middle of winter in the temperate zones. Summer, a very different story.
Some researchers have made the connection (many decades ago) but to most the cause of this seasonal pattern in infection rates is still a complete mystery. For some reason.
The vitamin D theory is interesting but I think very much a secondary influence. The seasonal variability in indoor ventilation and infectious particle density explanation being much simpler and more straight forward.