The number of available COVID-19 vaccine doses is steadily rising around the world.
But a shortage of physical space that meets standards for pharmaceutical manufacturing is a becoming a bottleneck.
That's according to drugmakers, construction experts and officials involved in the U.S. vaccine program.
The production of raw materials, vaccine formulation and vial filling requires special "clean rooms".
They need features like air cleaners, sterile water and sterilizing steam.
Moderna this week announced plans to expand vaccine manufacturing capacity.
But said it will be a year before that can add to its production.
With vaccines needed for billions of people, drugmakers have even had to turn to rivals for help in churning out doses.
And the emergence of new variants is likely to increase the strain.
Many are counting on the authorization of Johnson & Johnson's vaccine this week.
Longer term, tackling COVID-19 may require annual shots to protect against new virus mutations, similar to the flu.
Building new facilities and even expanding existing sites has typically taken years.
During the pandemic, some projects have reportedly been completed in as little as 6-to-10 months.
Emergent BioSolutions, which is making J&J and AstraZeneca vaccines for the US, says it cannot add any more equipment to facilities dedicated to those vaccines.
Some firms are purchasing and repurposing existing plants to sidestep construction.
Pfizer-partner BioNTech bought a German facility from Novartis in September.