To better understand breakthrough infections, researchers said that it is important to know if the vaccines were properly stored, the underlying conditions and behavior of the "hosts" who got infected and the nature of the virus that infected them. Were the people taking certain medications that blunted the effectiveness of the vaccines? Were they vaccinated at a time when they had little sleep or had been drinking excessively? Swartzberg asked. Could those factors interfere with the vaccine? "It is pure speculation," he said. He said the amount of exposure to the virus also may play a factor, overcoming the immunity one gets from the vaccine. "The dose makes the poison," he said. "In many cases with infectious diseases, the more virus you get exposed to in one setting, the more likely you are to be infected."