SYDNEY, July 19 (Xinhua) -- The air delivery of crucial polio vaccine supplies to Papua New Guinea's northern Morobe province is being hampered by bad weather, amid efforts to improve the immunization rate following a recent outbreak of the disease, according to local health authorities.

The vaccine supplies are ready for distribution but heavy rain and cloud cover could hamper their delivery on time, with at least five areas affected, deputy provincial health adviser and polio response management operations coordinator Jack Aita was quoted by local media as saying on Thursday.

Boxes of vaccine are being stored in holding facilities with the inclement weather preventing helicopter landings at destination sites, while at least one flight has had to be turned back because of storms and cloud cover, local media reported.

The delays will not affect the polio vaccines themselves, which are stored in cooling boxes and placed in freezers in the meantime, said Aita.

Authorities on Monday rolled out mass polio vaccination in the Lae and Huon Gulf districts as part of efforts to ramp up the vaccination rate against the disease, which is known to cause lifetime paralysis in children, following a recent outbreak that has since been brought under control.