Some population groups may already be close to achieving “crowd immunity”, despite more or less strict social distancing measures.
An international group of scientists suggested that the herd immunity threshold (HIT) in the new coronavirus may be much lower, not 60-70%, but 10-20% of immune people, and perhaps even less. The fact is that people are not equally susceptible to SARS-CoV-2, and the “weaker” groups are quickly depleted, which slows down the outbreak.
The numbers are quite below the minimum coverage required to quell the epidemic by accidental vaccination, which for a virus with a reproductive index above 2.5 is estimated at 60% and above.
The authors believe that the main reason for the inapplicability of the classical formula is that more susceptible and connected people have a higher tendency for infection and, thus, may become immune earlier than others. Because of this natural selective immunization, heterogeneous populations need fewer cases to achieve HIT than is suggested by models that do not fully account for variation.
Scientists believe that the main reason for the different sensitivity of people to the new coronavirus is that a significant part (from 35% to 80%) of those who are not yet infected with SARS-CoV-2 have T cells, antibodies or other components that appeared in them as a result of a cold suffered in the past due to other “harmless” coronaviruses or flu.
That is, many people already have cross-reactive immunity of varying strength to the new pathogen. In other words, many of us can fight COVID-19 in one way or another. Since such antibodies are not specific for SARS-CoV-2, and T cells are difficult to detect at all, these components of immunity are usually not detected by current tests for immunity to the new coronavirus, scientists write.
They note that despite the weakening of various quarantine measures, in some regions there is no significant increase in deaths, and this may be evidence of the correctness of their hypothesis.