Dr Anthony Fauci has warned that "breakthrough" infections of Covid-19 among individuals who are vaccinated are bound to occur.
The Director of the National Institute of Allergy and Infectious Disease said there could be hundreds or even thousands of instances of "breakthrough" infections, and that such infections occur with all vaccines, not just the coronavirus vaccine.
Dr Fauci said it was important to compare the relative low number of breakthrough infections against the tens of thousands - and eventually millions and hundreds of millions - of effective vaccinations.
"We see this with all vaccines, in clinical trials, in the real world," Dr Fauci said at a White House briefing. "No vaccine is 100 percent efficacious, or effective, which means that you will always see breakthrough infections, regardless of the efficacy of your vaccine."
He pointed to the flu vaccine, which is only between 40 and 60 per cent effective, and said breakthrough infections also occur with that vaccine.
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Despite breakthrough infections occurring, Dr Fauci said that often if a vaccine does not fully stop an infection, it still protects against serious symptoms that may have otherwise arose due to the disease.
"If you get vaccinated, no doubt, you're less likely to get the flu. But even if you do get the flu and get sick, vaccination can reduce the severity and duration of illness, and could help get you out of trouble," Dr Fauci said.
He also said that the US may reach some sense of "normality" by the end of the summer.
"It's very difficult to predict, but I would think that we would approach some degree of normality as we get towards the end of the summer and into the fall, and a considerable degree of normality as we get into the winter of this coming year," he told Business Insider.
He said that prediction operates on the belief that the overwhelming majority of people get the vaccination and that those who are vaccinated also do not carry the virus.
