Novavax Inc.'s shares plunged nearly 20% in premarket trading on Tuesday due to uncertainty over global demand for the company's COVID-19 vaccine following a slow start to deliveries.

The company said in September it had targeted for the delivery of at least 2 billion doses of its COVID-19 vaccine in 2022. Novavax said on Monday it delivered about 42 million doses globally in the first quarter.

Serum Institute of India, the world's biggest vaccine maker, also produces the company's COVID-19 vaccine under the brand name Covovax.

Novavax continues to see low vaccination rates across low-income countries, Chief Business and Commercial Officer John Trizzino said on a conference call.

"To date, we have not yet received an order from Gavi and the timing and quantities of future orders to deliver 2373 [the vaccine] to the COVAX facility are unclear," Trizzino said.

COVAX, a program aimed at equitable access to COVID-19 vaccines, is backed by the World Health Organization and global vaccine alliance Gavi.

Novavax's COVID-19 vaccine is based on a more conventional technology and was expected to persuade some skeptics of the mRNA technology used by companies such as Pfizer and Moderna.

However, Novavax has been plagued by manufacturing and regulatory delays, as well as a slow uptake in key markets such as the European Union.

The company on Monday reiterated its forecast for total revenue of $4 billion to $5 billion for 2022, saying it expects shipments to key markets and sales to increase in the second quarter.