为什么很难制造疫苗和增加供应

Demand for COVID-19 vaccines is far greater than the world's supplies. Everyone from world leaders to policymakers to the general public wants to know: How and when can we get more?

The answer is not easy. Makers of COVID-19 vaccines need everything to go right as they increase production to hundreds of millions of injections. Any problem could cause a delay. Some of the ingredients used in the vaccines have never been produced in such huge amounts before.

In the words of vaccine specialist Maria Elena Bottazzi, "It's not like adding more water to the soup." Bottazzi works at Baylor College of Medicine in Texas.

The many different kinds of COVID-19 vaccines being used in different countries all train the body to recognize the new coronavirus -- mostly the spike protein that covers the virus. But they all require different technologies, raw materials, equipment and expertise to do so.

The two vaccines currently approved for use in the United States, from Pfizer and Moderna, are made by putting a piece of genetic material called mRNA inside a small amount of fat.

Making small amounts of mRNA in a research lab is easy. But, "nobody made a billion doses or 100 million or even a million doses of mRNA" before the health crisis said Dr. Drew Weissman. He is with the University of Pennsylvania and helped develop mRNA technology.

Increasing production does not just mean getting more ingredients. Creating mRNA involves a chemical reaction between genetic materials and chemicals called enzymes. Weissman notes that the enzymes do not work as well in larger amounts.

The vaccine developed by AstraZeneca and another one expected soon from Johnson & Johnson are made with a cold virus that sneaks the spike protein gene into the body. This requires a very different form of manufacturing. For these vaccines, living cells in huge bioreactor devices grow the cold virus. The virus is then removed and purified.

"If the cells get old or tired or start changing, you might get less," Weissman said of the process. "There's a lot more variability and a lot more things you have to check."

More traditional vaccines, such as the one made by China's Sinovac, require even more steps and greater biosecurity. That is because they are made with killed coronavirus.

However, all COVID-19 vaccines must all be made using strict rules that require specially inspected facilities. Manufacturers also must test each step in the process often.

What about supplies?

Production depends on having enough raw materials. Pfizer and Moderna have promised they have dependable suppliers.

Even so, a U.S. government spokesman said planning experts are working directly with vaccine makers to help solve any problems that come up.

Moderna chief executive Stephane Bancel admits that difficulties remain. Bancel said if "there's one raw material missing, we cannot start making products and that capacity will be lost forever because we cannot make it up."

Pfizer has temporarily slowed deliveries in Europe for several weeks so that it could improve its factory in Belgium to deal with increased production.

AstraZeneca has also informed European Union leaders that it, too, will deliver fewer doses than promised. The company said it has seen lower than expected production at some European manufacturing centers.

How much is on the way?

Many countries want to know how many vaccine doses they can expect -- and by when.

Moderna and Pfizer each plan to deliver 100 million doses to the United States by the end of March and another 100 million within the next three months. President Joe Biden has announced plans to buy more supplies over the summer. The goal is to have enough to vaccinate 300 million Americans.

Pfizer chief Albert Bourla said at a conference recently that his company will actually be able to provide 120 million doses by the end of March. The reason is not because of faster production. It is because health workers are now permitted to get an extra dose out of every container of vaccine.

But getting six doses instead of five requires using specialized syringes -- the devices used to give shots of vaccine. There are questions about the worldwide supply of such syringes.

Moderna also recently announced it will be able to supply 600 million doses of vaccine in 2021. That is up from an earlier estimate of 500 million.

The easiest way to get more doses, however, is through the approval of other vaccines in development. U.S. data on the effectiveness of Johnson & Johnson's one-dose injection is expected soon. Another company, Novavax, also is in its final period of testing.

Other options

The Serum Institute of India has a contract to manufacture a billion doses of AstraZeneca's vaccine. It is the world's largest vaccine maker and is expected to be an important supplier for developing countries.

But some other efforts to increase supplies appear to be facing problems. Two Brazilian research institutes have plans to make millions of doses of the AstraZeneca and Sinovac vaccines. But those have been set back by unexplained delays in shipments of ingredients from China.

I'm Jonathan Evans.

And I'm Jill Robbins.

Ashley Thompson adapted this story from the Associated Press. Mario Ritter was the editor.