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Daiichi Sankyo Co.’s home-grown COVID-19 vaccine, a cutting-edge mRNA shot that’s on the cusp of its final clinical trials, could be used mainly as a booster starting next year for people who have already been immunized, the drugmaker said.

While inoculations from companies including Pfizer Inc.-BioNTech SE, Moderna Inc. and AstraZeneca Plc. are being rolled out across the globe, many others are still in development. Manufacturers including Daiichi are targeting local markets that don’t have an ample vaccine supply or are anticipating future needs, including booster shots.

"The most likely scenario is that most people in Japan would’ve gotten one of the already-approved vaccines by next year,” said Shizuko Ueno, the project leader for COVID-19 vaccine development at Daiichi Sankyo. "We expect that it could be used as a third booster shot and are looking into running a trial for that as well.”

Japan, which has vaccinated nearly half of its population, has deals with Pfizer, Moderna and AstraZeneca for enough shots to cover all its residents. The country has also made additional purchases for shots in anticipation of needing a third dose, which may start next year.

The timeline could be a hurdle for Daiichi, which is developing the shot with the University of Tokyo. It’s aiming for full regulatory approval by the second half of next year. Japan’s regulators would have to clear it as a primary shot before considering it as a booster dose, Ueno said in an interview. The company is working closely with the Pharmaceutical and Medical Devices Agency, which advises the health ministry on approvals, she said.

The island nation currently doesn’t allow mix-and-match vaccinations, though the government has indicated it wants to consider it. Research has shown that giving people follow-up vaccines that work differently than their original dose may yield a stronger and more protective immune response.

Daiichi, one of Japan’s largest drugmakers, is best known for its cancer treatments and has made several high-profile development agreements in recent years with AstraZeneca. Daiichi is also helping manufacture AstraZeneca’s vaccines in Japan.

The final trial for Daiichi’s vaccine will involve thousands of people across multiple countries, though it hasn’t yet started enrolling, Ueno said. It will directly compare the novel shot to another vaccine already on the market, reflecting a shift in trial design since it’s no longer considered ethical to test an active COVID-19 shot against a placebo.

Two other head-to-head studies currently underway use AstraZeneca’s shot as a basis for comparison, including one from GlaxoSmithKline Plc and South Korea’s SK Bioscience Co.

"We haven’t decided which to use yet for comparison,” Ueno said. "There’s different schools of thought about whether we should use the Pfizer or Moderna, which are similar mRNA vaccines, or use the AstraZeneca.”

Japan’s pharmaceutical industry has been slow in developing a home-grown vaccine, but companies like Daiichi, Shionogi & Co. and AnGes Inc. are still investing in the effort, anticipating domestic demand and eyeing future markets overseas.

Daiichi’s mRNA vaccine works in a similar way to Pfizer’s and Moderna’s shots, two of the most effective immunizations now on the market. But Daiichi’s shot will have a more specific mechanism of action. It will prompt the body’s healthy cells to make a specific part of the coronavirus’ spike protein, called the receptor binding domain, said Masayuki Yabuta, an executive officer for the company’s biologics division.

Since the receptor binding domain doesn’t change as the virus mutates, the idea is that it may retain its effectiveness against future variants, Yabuta said.

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