Forgot to indicate that Singapore is in the Equatorial Asia zone, dissimilar to Malaya (and the rest of the Malaysian Federation) being "year-round" influenza zones. It's apparent to me that the WHO guidelines are delineated by countries, rather than geography. Is it due of a dearth of studies?

The Philippines, whose southern islands are in close proximity to Borneo, is part of the Tropical Asia vaccination zone (alongside most of Mainland Southeast Asia and Timor-Leste.) In spite of being latitudinally above Malaysia and Indonesia, the Southern formulation is recommended in this zone.