The shortages are the result of COVID-19 vaccine campaigns, with billions more syringes than normal being used worldwide, badly denting global supplies.
Lisa Hedman, the WHO's senior advisor on access to medicines and health products, said as the supply of COVID-19 vaccine doses increases, the supply of syringes needs to keep pace.
"We are raising the real concern that we could have a shortage of immunization syringes, which would in turn lead to serious problems, such as slowing down immunization efforts," she told reporters.
"Depending on how the vaccine uptake goes, it could be a deficit of anywhere from 1 billion to 2 billion."
More than 7.25 billion COVID-19 vaccine doses have been administered globally, according to an AFP tally.
That's nearly double the number of routine vaccinations given per year – and twice the number of syringes required.
Hedman said one serious result of a shortage could be delays in routine vaccinations, which could have a public health impact "for years to come" if a generation of youngsters miss out on normal childhood vaccinations.
Shortages could also lead to the unsafe practice of reusing syringes and needles.
Hedman said any constraint on syringe supply could only be worsened by export restrictions and transportation problems.
She urged countries to plan syringe needs well in advance to avoid hoarding and panic buying situations.
Hedman said "efforts are being made to reduce that risk to zero in terms of the actual number that we could be short".