WASHINGTON: US biotech firm Novavax on Monday said it had delayed plans to seek authorization for its Covid vaccine candidate, a setback for developing countries hoping the injection would soon be available.

The company now hopes to file for authorization in the US and Europe in the third quarter of 2021, it said in a statement, sending shares tumbling 12 percent.

Novavax's vaccine is key to the Covax program, which was set up to try to ensure poor countries have equitable access to vaccines to combat the coronavirus pandemic.

Its NVX-CoV2373 vaccine candidate demonstrated 100 percent protection against severe disease in a Phase 3 clinical trial in Britain, with 89.7 percent efficacy against any form of the disease.

It uses a piece of the SARS-CoV-2 virus called the spike protein, which the company mass-produces inside insect cells, and can be stored at regular refrigeration temperatures.

Novavax also said Monday it had had positive results from preclinical studies of a shot combining its influenza and Covid vaccine candidates.

The company said a study, which will now undergo peer review, showed that the shot produced strong levels of antibodies against both diseases in ferrets and hamsters.

"Despite low rates during the Covid-19 pandemic, influenza remains a significant risk to global public health and the need for versatile, more effective vaccines is as important as ever, including against the flu," said Gregory Glenn, the company's president of R&D.

The new vaccine combines NanoFlu and NVX-CoV2373.

Hamsters that were vaccinated and then deliberately infected with the coronavirus retained their body weight compared to those that received a placebo.

Lung samples taken from the immunized animals after they were put down showed they were Covid-free.