The first animal studies trickling out show no sign of a side effect that has worried researchers. But it’s still not clear exactly how much of an immune reaction possible vaccines need to spark for real protection. (June 2) Subscribe for more Breaking News: http://smarturl.it/AssociatedPress Website: https://apnews.com Twitter: https://twitter.com/AP Facebook: https://facebook.com/APNews Google+: https://plus.google.com/115892241801867723374 Instagram: https://www.instagram.com/APNews/ ​ You can license this story through AP Archive: http://www.aparchive.com/metadata/youtube/8eb64c80abcd40198bbbe0a403a6d1f5