The key to herd immunity is that, even if a person becomes infected, there are too few susceptible hosts around to maintain transmission; those who have been vaccinated or have already had the infection cannot contract and spread the virus. A vaccine’s ability to block transmission doesn’t need to be 100% to make a difference. Even 70% effectiveness would be “amazing” says Samuel Scarpino, a network scientist who studies infectious diseases at Northeastern University in Boston, Massachusetts.

Calculations for herd immunity consider two sources of individual immunity; vaccines and natural infection. People who have been infected with SARS-CoV-2 seem to develop some immunity to the virus, but how long that lasts remains a question says Shweta Bansal, a mathematical biologist at Georgetown University in Washington DC. Given what is known about other coronaviruses and the preliminary evidence for SARS-CoV-2, it seems that infection-associated immunity wanes over time, so that needs to be factored into calculations. “We’re still lacking conclusive data on waning immunity, but we do know it’s not zero and not 100,” Bansal said.

A summary of the study is below. To review the entire study, click the link here.

• Every adult who wants to get a vaccine will be able to get a vaccine by May/June 2021.
• The US will be near COVID-19 herd immunity by summer 2021 (Jun-Aug 2021). At a high level, herd immunity is a concept in which a population can be protected from a virus if enough people possess immunity. Current accepted estimates for the herd immunity threshold range from 60-90% of the population.
• Due to vaccine hesitancy and the later arrival of a children’s vaccine, it is possible that we do not reach the levels required for herd immunity in 2021. Nevertheless, herd immunity is not a hard threshold, and being close to herd immunity may be sufficient to prevent large outbreaks.
• At the time herd immunity is near, roughly half of the immunity will be achieved through natural infection, and the other half will be achieved through vaccination.
• New infections may become minimal before herd immunity is reached. But due to imported cases and localized clusters, it is unlikely that new infections will drop to zero until at least 2022.
• Deaths may drop to low levels even earlier (May-Jul 2021), in part due to a vaccine distribution strategy that initially prioritizes high-risk individuals. Once deaths fall to minimal levels, we may see a relaxation of restrictions.
• Summarizing the above findings, our best estimate of a “return to normal” in the US is summer 2021 (June-August 2021). We define this as the removal of all restrictions for the majority of US states.
• We estimate roughly 60-70% of the US population (~220 million) will receive at least one dose of the vaccine by the end of 2021, with children being the last group to receive it (starting in summer 2021 or later).
• We estimate around 30-35% of the US population (100-120 million) will have been infected by the SARS-CoV-2 virus by the end of 2021. That is an additional ~50 million infections since mid-December 2020.
• This translates to a final US COVID-19 death toll of roughly 600,000 (±100,000) reported deaths by end of 2021, or ~300,000 additional deaths since mid-December 2020.

Stay well, roll up your sleeve and stay tuned!