In the United States, the distribution of the Johnson & Johnson (J&J) COVID-19 vaccine has been slower than expected, with over 6 million doses delivered to states in the first month.

Now, The New York Times has reported that there has been an unfortunate mix-up in the production process.

The news outlet reported that “workers at an Emergent BioSolutions plant in Baltimore mixed up vaccine ingredients weeks ago and ruined a large batch of the vaccine containing around 15 million doses,” according to Fierce Pharma.

The U.S. Food and Drug Administration (FDA) is investigating the error, but the incident left a temporary halt to future J&J vaccine deliveries from the site.

J&J said in a statement that it has been working with federal officials to deliver 24 million doses of the vaccine in April.

According to The Times, federal investigators have ruled that the episode was the result of “human error.”

The company said it is now moving to implement tighter controls at its manufacturing partner. The company said Emergent spotted the issue and notified the FDA.

J&J said its efforts to manufacturing ramp-up “include test runs and quality checks to ensure manufacturing is validated and the end product meets our high-quality standards.”

While J&J and Emergent seek an FDA manufacturing authorization for the site, “J&J is providing additional experts in manufacturing, technical operations and quality to be on-site at Emergent to supervise, direct and support all manufacturing of the Johnson & Johnson COVID-19 vaccine.”

The company plans to ship 24 million doses of the vaccine in April.

In November 2020, AstraZeneca acknowledged a manufacturing error that raised questions over the preliminary results of its COVID-19 vaccine.

Meanwhile, 98 million Pfizer-BioNTech vaccine doses have been delivered and 90 million Moderna vaccine doses have been sent, accruing to CDC. Moderna has recently said it has shipped 100 million doses to the government so far. The story was published Wednesday on Fierce Pharma.