Phase four means that a virus has gained the ability to cause community-level outbreaks.

VOA: special.2009.05.06

There's a lag phase, there's a slow rise to an intermediate level of antibody.

会有迟滞期,在该时期,抗体浓度缓慢到达中间水平

耶鲁公开课 - 生物医学工程探索课程节选

The World Health Organization says it needs to see clear evidence of sustained community transmission of the swine flu virus from person-to-person in at least two regions of the world before it raises its alert to the phase six pandemic level.

VOA: standard.2009.05.14