This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Mercedes Yanes Lane

Researchers in Brazil repeatedly measured antibodies against SARS-CoV-2 in blood donors, and found that in the Amazon, close to half of donors were positive for antibodies (1) The authors of this study found that there was a steady drop in antibodies over time, although the number of cases continued to rise. They attributed this drop to the natural decline in immunity, and when using mathematical formulas to account for this decline, they found that up to 76% of the population may have been previously infected in Manaus, capital of Amazonas state, by October of 2020. This high infection record surpasses the required number to reach the theoretical herd immunity threshold of 67%.

The term ‘herd immunity’ refers to the the resistance to the spread of a contagious  disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, either through vaccination or by a previous natural infection.

Nevertheless, an abrupt increase in the number  of COVID-19 hospital admissions in the region in  the  beginning of 2021 raised significant concern (2) There are four possible non-exclusive explanations for the resurgence of COVID-19 in the region. First, infection rates may have been over-estimated, meaning herd immunity was never reached in 2020. Second, immune protection after natural infection may not last long,     as most of the SARS-CoV-2 infections from the first wave in the region occurred 7–8 months before the resurgence in January 2021. Third, new circulating variants of SARS-CoV-2 may escape  the  protective  immunity acquired from a previous infection. Fourth, new circulating variants of SARS-CoV-2 may be more infectious that its predecessors, increasing the theoretical herd immunity  threshold.  The  P.1  variant,  that was first discovered in Manaus, shares several  independently  acquired  mutation  variants  circulating  in the UK and South Africa, which seem to have increased transmissibility. Determining the efficacy of existing COVID-19 vaccines against variants in the P.1 lineage that can potentially escape natural immune protection would be important to assess the need for new interventions.

In contrast, reports from Israel show that vaccination  against  COVID-19  is helping reduce  the number  of cases and hospitalizations. Vaccines, together with other public health measures such as  lockdowns,  are helping to reduce the spread of the disease in a country where close  to 90% of people  ages  60 and above have received the first dose  of the Pfizer-BioNTech  vaccine (3) Data from the Ministry of Health, encompassing almost a quarter of a million people vaccinated aged 16 and older, shows that 13 days after vaccination, there is a drop of 51.4% in the number  of cases compared to the first 12  days  post-  vaccination3. These findings indicate that the vaccine is effective in preventing infection  across  all  age groups, with a drop in cases ranging from 44.5% in people older than 60 years,  to 50.2% in people  younger  than 60 years (4) However, researchers caution the need for widespread vaccination in order to be able to effectively measure the reduction in transmission.  In order to  reduce  transmission, COVID-19  vaccines should reduce viral shedding. Investigators from My Heritage Lab, the largest COVID-19  testing  facility  in Israel, reported that vaccination indeed significantly reduced the amount of virus carried by SARS-CoV-2 infected individuals (5). Altogether these results indicate that vaccination not  only protects the recipient of the vaccine, but it could also significantly reduce transmission.