A World Health Organization official this week warned that a COVID-19 vaccine is not necessarily a "silver bullet" for ending the pandemic.

"There is some light at the end of a long tunnel but these vaccines are not a silver bullet that will end the pandemic in the near future," Takeshi Kasai, WHO’s regional director for the Western Pacific, said on Thursday, per the South China Morning Post. 

"This means that tired as we all are of this pandemic, we must stick to the actions and behaviors which protect not only ourselves but also those around us: hand washing, mask-wearing, physical distancing and avoiding places that have a high risk of transmission," he added. 

The WHO has recommended that high-risk groups – those who are 60 years of age or older and those with underlying health issues – be the priority for the initial doses of the vaccine. Kasai noted that if the "right scale and type of investment are made," there should be "adequate doses to vaccinate a high-priority population in all countries around the world," by the end of 2021. 

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"For others, beyond those high-risk groups, we may be looking for another 12 to 24 months before the majority of people have received this vaccine, and even then, there is some uncertainty and unknowns," he continued. 

In the U.S., Dr. Anthony Fauci, the nation’s top infectious disease expert, said on Monday that he expects the country to see herd immunity by late spring or early summer after coronavirus vaccines become more widely available.

"By the time we get to the fall, we can start approaching some degree of relief where the level of infection will be so low in society we can start essentially approaching some form of normality," he said, though noting that mask-wearing and other precautionary measures will likely continue "several months into the second half and beyond of 2021."

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Right now, the FDA has granted emergency approval to a COVID-19 vaccine developed by Pfizer and BioNTech. Health care workers and residents and staff of long-term care facilities began to receive the jab this week. Another promising candidate developed by Moderna is likely to gain emergency use approval as early as this week. 

A third candidate, from Johnson & Johnson, which would require just one dose, is working its way through the pipeline. Behind that is a candidate from AstraZeneca and Oxford University. U.S. health experts are hoping a combination of vaccines will ultimately enable the U.S. to conquer the outbreak.

The Associated Press contributed to this report.