Sanofi and GlaxoSmithKline Plc, two of the world’s biggest vaccine makers, delayed advanced trials of their experimental COVID-19 shot after it failed to produce a strong enough response in older people, pushing its potential availability to the end of next year.

The partners will begin a new second-phase study with a more concentrated antigen in February after they said the current dosage failed to generate a good immune response in people 50 years and older. Younger adults showed a response similar to patients who have recovered from the disease.

The delay underscores the difficulties and uncertainties companies face in developing shots against a disease that’s already claimed more than 1.5 million lives. It’s also a blow for governments that were counting on supplies from the two vaccine giants amid expectations the world will need multiple inoculations to stop the spread of the pathogen.

In another setback for vaccine developers, Australia canceled an order for 51 million doses of a COVID-19 shot being developed by CSL Ltd. and the University of Queensland. A component of the vaccine comes from the human immunodeficiency virus, and while that posed no risk of infection, some trial participants had false positive tests for HIV.

Pfizer, Moderna

The announcements temper some of the optimism following positive trial results from Pfizer Inc. and Moderna Inc. Another inoculation from AstraZeneca Plc and the University of Oxford looks promising, too, despite questions about its effectiveness in older adults. The U.K. and Canada have already approved a shot from Pfizer and partner BioNTech SE that employs messenger RNA technology, and the U.S. and Europe may do so soon. China and Russia have already begun administering their own vaccinations.

Sanofi fell as much as 2.4 per cent early Friday in Paris, with Glaxo down as much as 0.7 per cent in London.

The delay to the Sanofi-GSK vaccine means it may not reach the market until the fourth quarter of 2021 rather than the middle of the year. A study using an improved antigen mix, exposing monkeys to the virus, showed a rapid clearance of the pathogen from the nose and lungs.

The companies are planning a new trial that could include a head-to-head comparison with an approved COVID-19 vaccine to speed research. The companies haven’t disclosed how many people will be included. Advanced-stage tests could start in the second quarter of next year.

This particular candidate relies on recombinant DNA technology Sanofi uses to make influenza shots, along with Glaxo’s adjuvants, which enhance the body’s immune response. The companies previously said they expected to produce as many as 1 billion doses in 2021. Sanofi has another mRNA shot for COVID in development and GSK is involved in a handful of other collaborations for COVID.

The recombinant DNA approach received approval in the U.S. for combating the flu in 2013. It’s a synthetic process that aims to produce an exact genetic match of proteins found on the surface of the virus.

What Bloomberg Intelligence Says

“This development may increase the odds of an approval for AstraZeneca’s vaccine despite unanswered questions over its trial and data.”

-- Sam Fazeli, senior pharmaceutical analyst

The delay highlights the challenges a number of second generation COVID-19 vaccines could face now that the first shots are being approved. One potential benefit of a comparator trial arm with another COVID-19 vaccine will be recruitment. With as many as three approved vaccines possibly available in the coming weeks, studies for those further back in the development process may need to rethink incentives to keep enrollment up.

The biggest orders for the vaccine include the European Union, which has agreed to buy 300 million doses, and the U.S., which is purchasing 100 million with an option for more. Sanofi and GSK have intended to provide 200 million doses of their vaccine to the Covax program, which is aimed at distributing shots equitably all over the world.

Glaxo is also developing vaccines with Sichuan Clover Biopharmaceuticals Inc. of China, which reported last week that the shot had induced neutralizing antibodies in early-stage trials. The U.K. company has also begun late-stage trials for a vaccine with Canada’s Medicago Inc.