Herd immunity against COVID-19: is it enough to stop a second wave?

To the Editor,

Humanity faces its greatest challenge in the past 100 years as the outbreak of coronavirus disease 2019 (COVID-19) has caused more than 17 million confirmed cases all over the world [1].

About 80% of COVID-19 cases are asymptomatic or present with mild symptoms, but some cases will progress to respiratory distress, which may necessitate mechanical ventilation [2]. The host immunological response plays an important role in determining the outcome of infection [3]. Individuals with severe COVID-19 usually have a higher level of interleukin-6, interleukin-10 and tumour necrosis factor-α, and fewer CD4+ and CD8+ T cells than individuals with mild disease [4].

Many reports have discussed the durability of post-infection immunity after COVID-19. The neutralizing antibodies that are produced by B cells after infection were found to be successful in preventing re-infection in animal models, and this was the concept behind using convalescent plasma as a potential therapy for COVID-19 [5].

Unfortunately, using convalescent plasma as a therapy for COVID-19 did not achieve success when added to the standard of care of individuals with COVID-19 in real-life studies. The short-lasting humoral immune response and the required high neutralizing antibody titre (above 1 : 640) are the main limiting factors in its success. These data raise suspicions concerning the efficacy of post-infection immunity and subsequently of herd immunity in controlling COVID-19 [6,7].

Using herd immunity as a defence strategy against COVID-19 has been adopted by many countries. Herd immunity refers to the indirect protection from infection conferred to susceptible individuals when a sufficiently large proportion of immune individuals exist in a population [8]. A trial of implementing this strategy was done in the United Kingdom at the beginning of the pandemic but it was not successful [9]. Additionally, countries who lifted their community preventive measures depending on herd immunity are beginning to report a second rise of infections. Studies showed that the estimated proportion of the total population infected varies between countries from less than 1% in Norway, Germany and Austria to 8% in Belgium, which is too low to achieve herd immunity [10].

Lifting of COVID-19 community preventive measures makes these countries vulnerable to the so-called ‘second wave of COVID-19’ [11]. Countries should not rely on herd immunity for protection against the second wave of COVID-19. Effective community preventive measures with optimization of screening and treatment are the cornerstones for success.

Conflicts of interest

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. The authors declare that there is no conflict of interest’.