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The Trump administration is studying the safety of an ingredient commonly used in vaccines, an additive that contains aluminum. Coming after other recent changes to vaccine policy , many public health experts worry that the administration may now try to remove the ingredient.

"We want no aluminum in the vaccine," President Trump said recently at a White House briefing.

For almost a century, some important vaccines, including shots that protect against diphtheria, tetanus, hepatitis and the flu, have included aluminum salts, compounds that contain small amounts of aluminum . They are used as adjuvants to give the immune system an extra kick necessary to make the shots protective.

"Aluminum is added to some vaccines because it's a

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