People on six continents are already getting jabs as the race for a COVID-19 vaccine enters a defining summer.
The U.S. is poised to start even bigger studies next month to prove if any shot really works, a challenge as researchers chase a virus that's waxing and waning around the globe.
Already British- and Chinese-made vaccines are being tested abroad.
And experts want to set public expectations: Even if first doses of a vaccine arrive by year's end, don't expect perfect protection — and do expect a long line as countries prioritize who gets vaccinated first.
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