As it promised, GlaxoSmithKline is boosting production to help meet demand for its blockbuster vaccine against shingles, the painful aftereffect many older adults suffer decades after childhood chicken pox.

The company on Wednesday announced a $100 million investment in its manufacturing site in Hamilton, Mont., to expand production of key components of several vaccines, including Shingrix.

In January, GSK got U.S. Food and Drug Administration approval to use a manufacturing facility in France to make more shingles vaccine.

Shingrix, a potential $1 billion-a-year drug, was approved by the FDA in October 2017 to ward off shingles in adults 50 and older. The two-shot immunization is 90 percent effective at preventing the illness and its excruciating rash. Although the chicken pox virus goes dormant in nerve cells, it can be reactivated decades later; people who live to 85 have a 50-50 chance of developing shingles.

The original shingles vaccine, Merck’s Zostavax, never really caught on after its 2006 debut because it wasn’t very effective, but GSK’s two-shot immunization is extremely popular. Pharmacies in the Philadelphia area and around the country continue to have a hard time keeping it in stock. Even the hefty co-pay for some Medicare beneficiaries — it can top $150 for each of the shots — has not been a big deterrent.

More than nine million people worldwide have gotten Shingrix since it launched, according to GSK spokesperson Sean Clements.