Will We Be Able to Achieve Herd Immunity Against COVID-19?

[This document is a one of a series of discussion notes addressing fundamental questions about the COVID-19 crisis and response strategies. These documents are based on the best scientific information available and may be updated as new information comes to light.]

Written by Clara Marín, Oriana Ramírez, Carlota Dobaño, Jeffrey V Lazarus, Gemma Moncunill and Adelaida Sarukhan (ISGlobal), the document analyzes if Spain will achieve the herd immunity against COVID-19.

On 10 May 2021, the Spanish prime minister announced that, if the vaccine delivery schedule is met, Spain would reach the government’s desired level of herd immunity—70% of the population vaccinated—in 100 days, i.e. on 18 August. Given the available evidence, is this claim realistic?

The document addresses what percentage of the population must be vaccinated to achieve herd immunity. This is a threshold not known with any certainty because it is based on the virus’s basic reproduction number or R0, and the Ro of SARS-CoV-2 is an estimate: we do not know for sure how it responds to factors such as space or different variants of the virus.

The document addresses as well the obstacles to achieving herd immunity:

1. Although vaccines have been shown to reduce transmission, the available evidence suggests that they do not stop it completely.
2. Global vaccine distribution is very heterogeneous and in a globalised world herd immunity cannot be guaranteed for anyone unless a global health strategy is adopted.
3. More transmissible variants of the virus have emerged, pushing up the vaccination threshold required for group protection.

To complete the analysis, the document presents the experiences of Israel, Chile, India and Vietnam.

And concludes with some recommendations:

• Keep using non-pharmacological preventive measures
• Don’t leave anyone behind
• Aim for the highest possible vaccination coverage
• Do not disregard the functional control strategy. For some diseases, such as influenza, we will never achieve herd immunity

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