Because of T-cell immunity, it may only take 10 to 20 percent of a population to be infected with Covid-19 before herd immunity is reach. From Dr. James Todaro at theburningplatform.com:
1/ There is growing evidence that T-cell immunity allows populations to reach herd immunity once only 10-20% are infected with SARS-CoV-2.
This would explain why a highly transmissible virus in densely populated areas peaked at 10-20% infected regardless of lockdowns or masks.
2/ The pervasive misconception is that we have zero immunity against COVID-19. Based on this flawed understanding, epidemiologists projected that herd immunity is not reached until 60-70% are infected.
This is almost certainly wrong.
Of course, the media ignores this research
3/ While antibodies against COVID-19 may only last months, T cell immunity can remain protective for years.
In a study of 23 people who survived SARS in 2003, every single one had memory T cells that recognized the SARS virus 17 years later. (Nature)
4/ Moreover, blood samples from all 23 individuals showed “robust cross-reactivity” against SARS-CoV-2.
This can be called crossover immunity.
Crossover immunity is not limited to just people who were infected with SARS years ago though.
5/ In the same study, in 37 persons with no history of SARS or COVID-19 (negative serology and/or samples taken before COVID-19), over 50% had SARS-CoV-2 specific T cells.
This is not surprising because there are at least 4 strains of coronaviruses that cause the “common cold”.
6/ The above study is not the only one to show this level of cross-reactivity.
In a study from April 2020, in 68 healthy donors never exposed to COVID-19, 34% had T cells that reacted to SARS-CoV-2.