This year’s U.S. flu vaccine will be safe and will be distributed on time, Chiron Corp. President and Chief Executive Officer Howard Pien pledged on Tuesday.

Pien also said the company was working hard to develop a better and more efficient way to make influenza vaccine so it can adapt to epidemics that change as the virus mutates every year.

Last month Chiron said it would delay shipment of its FluVirin vaccine because lots containing 4 million doses of the vaccine did not meet sterility standards.

“We expect that in the next few days we can make shipment plans,” Pien told a hearing of the Senate Committee on Aging.

He said 48 million doses of the company’s vaccine, nearly half the total U.S. supply, would be shipped to distributors in October and November.

“This is only our second year as a supplier of flu vaccine to the United States but nonetheless we have increased output by 50 percent,” Pien said.

Influenza typically kills 36,000 Americans a year and 500,000 worldwide.

Pien said a few batches of the company’s vaccine being prepared for the upcoming flu season seemed to be contaminated, so the company halted production to check it out.

This created concern because last year there was a shortage of flu vaccine, but Pien said the checks were needed to guarantee that the vaccine was pure and safe.

“In our situation we determined that some small number of batches of product that were being made ... appeared to have contamination,” Pien told the hearing.

“Testing procedures and protocol calls for a program to confirm this. The first thing is to retest all of the product that we have made. The second is to look at all possible hypotheses as to why there might have been contamination.”

Then the root cause has to be found. Pien said the company was now working with the Food and Drug Administration to clear the vaccine for shipment.

“We hope that in due course, the next few days, we will make an announcement,” he said.

The formula for the influenza vaccine is changed every year to include the most common strains circulating at the time. It then takes months to grow the three strains in specially prepared chicken eggs and prepare the vaccine.

Last year 83 million doses were made. This year the Centers for Disease Control and Prevention says the companies that make flu vaccine will have 100 million doses available.

The CDC recommends that 185 million Americans get vaccinated against flu every year including the elderly, chronically ill, babies and their caregivers. But in reality not everyone does and often vaccine is thrown out at the end of the season.

Pien said Chiron was going to step up production.

“We will make the investment to the tune of about $100 million for this year and next to upgrade manufacturing capacity, to obtain more equipment and machinery, to train our work force even better,” he said.

“It is our deep and ardent commitment that we are going to do better and be a reliable and consistent supplier.”

Chiron shares were up 56 cents or 1.3 percent at $44.15 on the Nasdaq on Tuesday afternoon.