BERLIN—Germany, which has the world’s fifth-highest number of Covid-19 cases, has so far experienced far fewer deaths from the disease than other countries—mainly, experts say, because the outbreak started among younger people who tend to experience milder symptoms and rarely die.

Epidemiologists cautioned that the virus was likely to spread more widely among older people in the weeks ahead and that the mortality rate would likely rise, especially if hospital intensive-care units are overwhelmed.

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