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发布时间：2021-02-22 20:47:40 来源：听力鸭英语网
By Ashley Thompson
21 February 2021
The makers of coronavirus vaccines are learning how to change their vaccines to deal with virus mutations. Experts are looking to influenza, or flu, vaccines to help them understand if and when the shots may need to be changed.
Richard Webby directs a World Health Organization (WHO) flu center from St. Jude Children's Research Hospital in Virginia. He said that the process will not happen quickly.
"It's not really something you can sort of flip a switch, do overnight," Webby said.
Asnath Masango, gives a patient a COVID-19 test at the Ndlovu clinic in Elandsdoorn, 200 kms north-east of Johannesburg Thursday Feb. 11, 2021. The center is running a study of the Johnson & Johnson COVID-19 vaccine.
Viruses mutate, or change, often. Recent studies have raised concerns that first-generation COVID-19 vaccines are not as effective against a coronavirus mutation that first appeared in South Africa. However, many of the COVID-19 vaccines are made with new technology that permits changes. But it will still be difficult for experts to decide whether the virus has mutated enough to make it necessary to change the vaccines.
Flu offers a model
The WHO and the U.S. Food and Drug Administration (FDA) are looking to the worldwide flu vaccine system for help. Influenza mutates much faster than the coronavirus. Flu shots have to be changed nearly every year.
National centers around the world collect spreading flu viruses and observe how they change over time. They send samples to special WHO laboratories for high-level "antigenic" testing to decide on vaccine strength. WHO experts and government rule-makers work together to agree on each year's new vaccine recipe. Manufacturers then make the necessary changes.
For COVID-19 vaccines, Webby said an important step will be establishing a similar observation and testing network. It can then decide on which mutations are most worrisome. Today, there are wide differences in how nations track and test mutated versions. For example, Britain does more testing of the changing viral genome than the United States does.
Three variants first discovered in Britain, South Africa and Brazil are worrisome because of combinations of mutations that make them spread more easily and quickly.
On Sunday, U.S. researchers reported another mutation found in seven variants that have appeared in several U.S. states. Researchers do not yet know if this mutation makes the virus easier to spread.
Current COVID-19 vaccines and variants
Just because a variant spreads more easily does not mean it will be resistant to a vaccine. But the variant first identified in South Africa is raising concerns.
Columbia University's David Ho put blood samples from people given the Pfizer or Moderna vaccines into small containers with the mutated virus. He found that vaccine-produced antibodies still offered protection, but the protection was much less strong.
Early test results of two other vaccine candidates — from Novavax and Johnson & Johnson — supported Ho's findings. Both still offered protection. However, that protection was weaker in South Africa than in other places. A much smaller test of the AstraZeneca vaccine in South Africa has raised questions about its effectiveness.
Several of the approved and available COVID-19 vaccines require two shots for full effectiveness. The drugmaker Moderna is about to explore one possible solution: Could a third shot of its vaccine raise immunity enough to fight off some variants?
Ho said it is a good idea to test, because people may "still have plenty of cushion" if their antibody levels are very high.
Changing the recipe
Major manufacturers also are developing experimental variant vaccines as an extra measure.
COVID-19 vaccines produce antibodies that recognize the spike protein that covers the coronavirus. When the virus mutates, sometimes the spike protein is changed in ways that make it harder for vaccine-produced antibodies to recognize.
The Pfizer and Moderna vaccines are made with a piece of genetic information called messenger RNA (mRNA). It tells the body how to make some harmless copies of the spike protein that train disease-fighting cells. To change the vaccine, the drugmakers can simply change the first version of genetic information with mRNA for the mutated spike protein.
The AstraZeneca vaccine and the one made by Johnson & Johnson are expected to be available in the United States soon. Both are made with cold viruses engineered to introduce a spike protein gene into the body. Making changes to these two vaccines requires growing cold viruses with the mutated gene. The process is a little more complex than the mRNA process. But it is still not as difficult as remaking traditional flu shots.
Testing vaccines, round two
First-generation COVID-19 vaccines were tested in tens of thousands of people to be sure they work and are safe. The research took many months.
Simply changing the recipe to better target virus mutations will not require repeating those studies in thousands of people, said Dr. Peter Marks. He is the vaccine chief with the FDA. He recently told the American Medical Association that the FDA plans to be "pretty nimble" when it comes to approving vaccine changes. In other words, it will act quickly.
If an updated vaccine is needed, he said, tests in a few hundred people would likely be enough to know if it causes a good immune reaction.
I'm Ashley Thompson. And I'm John Russell.
Lauran Neergaard reported this story for the Associated Press. Ashley Thompson adapted it for VOA Learning English. Mario Ritter, Jr. was the editor.
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Words in This Story
mutation –n. a change in the genes of a plant or animal that causes a different quality to be recognized
flip a switch –idiom to easily and quickly make a change
sample –n. a small amount of something that is used to give information about what it was taken from
recipe –n. a set of directions for making food
track –v. to follow and observe, especially in an effort to find evidence
genome –n. the total amount of genetic information in the chromosomes of an organism
variant –n. something that is different in some way from others of the same kind
cushion –n. something extra that can reduce bad effects or problems
spike –n. a sharp, thin structure that sticks out from something