Herd immunity can’t be a "theory" since lack of it causes “second waves,” per Imperial College. If lockdowns prevent acquisition of sufficient herd immunity, a second wave of infection is possible once interventions are lifted-Neil Ferguson et al., 3/16/20

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March 16, 2020, “Report 9: Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand,” Imperial College London, Neil Ferguson et al.

page 7 of 20 (subhead, “Results,” page 6)

“The interventions need to remain in place for as much of the epidemic period as possible. Introducing such interventions too early risks allowing transmission to return once they are lifted (if insufficient herd immunity has developed); it is therefore necessary to balance the timing of introduction with the scale [page 8] of disruption imposed and the likely period over which the interventions can be maintained. In this scenario, interventions can limit transmission to the extent that little herd immunity is acquired –leading to the possibility that a second wave of infection is seen once interventions are lifted.”…

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