Covid-19 vaccine may have unpleasant side effects

The vaccine, and likely most others, will require two doses to work, injections that must be given weeks apart. Scientists anticipate that the shots will cause enervating flu-like side effects — including sore arms, muscle aches and fever — that could last days and temporarily sideline some people from work or school. And even if a vaccine proves 90 percent effective, the rate Pfizer touted for its product, 1 in 10 recipients would still be vulnerable. That means, at least in the short term, as population-level immunity grows, people can't stop social distancing and throw away their masks.

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