Studying herd immunity amid the COVID-19 pandemic is a great challenge for scientists as there is no city in the world that has managed to vaccinate the entire population – for now.

Pfizer Inc. will study the effectiveness of its vaccine against COVID-19 by inoculating the entire population over the age of 12 in a town in southern Brazil, the company said on Wednesday.

The study will be conducted in Toledo, population 143,000, in the west of Parana state, together with Brazil's National Vaccination Program, local health authorities, a hospital and a federal university.

Pfizer, which developed the vaccine with German partner BioNTech SE, said the purpose was to study the transmission of the coronavirus in a "real-life scenario" after the population has been vaccinated.

"The initiative is the first and only of its kind to be undertaken in collaboration with the pharmaceutical company in a developing country," Pfizer said.

A similar study was conducted by the Butantan Institute, one of Brazil's leading biomedical research centers, in the smaller town of Serrana, in Sao Paulo state. That trial tested the CoronaVac shot developed by China's Sinovac Biotech Ltd.

In May, Butantan said mass vaccination had reduced COVID-19 death by 95% in the town with a population of 45,644 people. The institute is considering extending the study for a third dose.

"Here we believe in science and we lament the almost 600,000 deaths from COVID-19 in Brazil," Toledo Mayor Beto Lunitti said at a news conference announcing the Pfizer study.

There is little anti-vaccine resistance in Toledo, where 98% of the population have had a first dose, mainly of the Pfizer/BioNTech vaccine. AstraZeneca's COVID-19 shot and Sinovac's have also been used there, municipal health secretary Gabriela Kucharski said, adding that 56% are fully vaccinated.

Regis Goulart, a researcher at Porto Alegre's Moinhos de Vento Hospital taking part in the study, said its aim was to validate the real-world efficacy and safety of the vaccine seen in clinical trials.

The observational study also will provide an opportunity to do long-term monitoring for up to one year of participants, helping to answer lingering questions such as the duration of vaccine protection against COVID-19 and new virus variants, Goulart said.