да я даже не про антитела, а про фактическую защиту.

Вот, нашла пример, исследование какого-то дома престарелых в США (задолго до дельты!):

Первая "волна", июль 20 года:

In total, 20 (17.4%) of 115 residents and five (3.5%) of 143 HCP in this facility received positive test results during July 16–August 11, representing an overall attack rate of 9.7%. Eight (40.0%) residents with COVID-19 were hospitalized, and five (25.0%) residents with COVID-19 died. No hospitalizations or deaths occurred among HCP with COVID-19.

Вторая волна, октябрь 20 года:

During October 30–December 7, a total of 85 (74.6%) of 114 residents and 43 (29.5%) of 146 HCP received positive SARS-CoV-2 RT-PCR test results, representing an attack rate of 49.2% among the 260 SNF residents and HCP present at the start of the outbreak in October. Among the 85 resident cases identified in the second outbreak, 15 (17.6%) patients died. No HCP died.

Among 12 residents who received positive test results during the first outbreak (July–August) and were still living in the facility in October, five also received positive results during the second outbreak >90 days after the date that their first specimens were collected.

https://www.cdc.gov/mmwr/volumes/70/wr/mm7008a3.htm