在美國麻疹疫苗接種率整體比較好， 2013年CDC報告顯示，年齡在19-35個月的兒童MMR疫苗接種率為91.9% ，但是各地區接種率差異還是很顯著的。

        例如最近麻疹疫情暴發科羅拉多州， MMR疫苗接種率隻有82%，CDC2013年報告顯示，美國有17個州的MMR疫苗接種率在90%以下。

        一份疾病預防控製中心的報告顯示，因為個人的原因，去年的麻疹病例中79%患者沒有選擇接種MMR疫苗。 盡管CDC表示MMR疫苗是安全的，單劑量疫苗的有效率約93%，而兩次劑量有效率約97%，但是為什麼還有很多人沒有給自己或孩子接種疫苗呢?

        首先，疫苗接種率下降是因為宗教或個人原因。大眾擔心這種疫苗對於兒童是不安全的。

        這種觀念來源1998年的一項研究，英國權威醫學雜誌《柳葉刀》發表醫生韋克菲爾德的論文，稱麻腮風三聯疫苗可能引發自閉症，經媒體報道後，不少家長拒絕為孩子接種該疫苗。雖然其他研究無法重現該結果，但該研究對疫苗接種影響顯著。英國此次麻疹暴發的起源應該追溯至此，6年後的2004年，英國三聯疫苗接種率由最高時期的92%降至81%。

        韋克菲爾德的論文發表後，不少醫學研究人員紛紛撰文批駁。英國醫學研究委員會等機構隨後也開展了長期研究，最終未發現這款疫苗與自閉症存在關聯。2010年，英國醫學總會吊銷了韋克菲爾德的行醫資格，《柳葉刀》也撤下了相關論文。

       Safety of MMR vaccine is still questioned

        While measles vaccination coverage across the US looks positive overall - with the CDC reporting a 91.9% MMR vaccine coverage among children aged 19-35 months in 2013 - it varies significantly by state.

        For example, Colorado - which has been affected by the latest measles outbreak - only has an MMR vaccine coverage of 82%, and a 2013 report from the CDC revealed that 17 US states have an MMR vaccine coverage below 90%.

        A report from the CDC found that last year, 79% of measles cases occurred among people who opted out of the MMR vaccine due to personal beliefs.The CDC say the MMR vaccine is safe, and one dose of the vaccine is around 93% effective at preventing measles, while two doses is approximately 97% effective. So why are many people still not getting themselves or their children vaccinated?

        Primarily, it is down to religious or personal beliefs. A report from the CDC found that last year, 79% of measles cases occurred among people who opted out of the MMR vaccine due to personal beliefs. And these personal beliefs tend to be associated with worries that the vaccine is unsafe for children.

        This belief stems from a study published in The Lancet in 1998, conducted by British researcher Andrew Wakefield and colleagues from the UK. In this study involving 12 children, Wakefield and his team suggested a link between the MMR vaccine and autism.

        Though other studies were unable to reproduce the findings, Wakefield's research had a significant impact on vaccination coverage. In the UK, MMR vaccination rates fell from 90% to 80%, while measles cases began to rise.

        The study has since been retracted by The Lancet, having been deemed as "fraudulent" after undisclosed financial conflicts of interest were revealed by a journalist called Brian Deer in 2011. As a result, Wakefield lost his medical license. But regardless of this outcome, he stands by his findings, and so do many others.

        One of the most well-known supporters of Wakefield's claim is former Playboy centerfold Jenny McCarthy, whose son developed autism, which she attributes to vaccination. She claims that current vaccines are unsafe and contribute to autism and other disorders.

        "I do believe sadly it's going to take some diseases coming back to realize that we need to change and develop vaccines that are safe," she said in an interview with the Chicago Sun Times last year. "If the vaccine companies are not listening to us, it's their [...] fault that the diseases are coming back. If you give us a safe vaccine, we'll use it."

        Many parents in the US support McCarthy's statement, but the CDC and other health organizations warn that this perception toward vaccination is putting the public's health at risk.