This is from the imdb faq. hope it helps

How did Anna get into Manhattan if the island was quarantined?
EditHistoryDelete While we do not know how the quarantine process was executed (possibly army/national guard troops), we can assume that 3 years following the deaths and/or mutation of 99% of humanity, the island is no longer being actively quarantined. One could easily hijack or build their own boat to gain access to Manhattan island

I put a link on where you can watch the entire movie

abcd, how does that help the situation?