Colorized scanning electron micrograph of an apoptotic cell (green) heavily infected with SARS-COV-2 virus particles (yellow), isolated from a patient sample. Image captured at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
In a new perspective published in The Journal of Infectious Diseases, Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases (NIAID), and colleagues essentially rule out herd immunity as a strategy against SARS-CoV-2, calling it an “elusive goal.”
Smallpox is the only human virus to have ever been eradicated, although mass vaccination efforts and other policies allowed the U.S. to achieve herd immunity against polio and measles.
Even so, long after herd immunity was achieved for measles and polio, the U.S. experienced both nationwide and localized outbreaks due to disease importation and enclaves of under-vaccinated populations.
“Time and again, human movement and other human behaviors have circumvented physical barriers between the infectious and the susceptible,” write NIAID authors Fauci, David Morens and Gregory Folkers. “Even global smallpox eradication had to contend with repeated outbreaks and cases among unvaccinated individuals throughout the decade leading to the very last natural case in 1978.”
In addition to these problems, which still exist in the 2020s, the barriers to achieving herd immunity are even greater with respiratory viruses such as influenza, RSV and SARS-CoV-2, as compared with measles and smallpox.
For example, SARS-CoV-2 appears not to substantially engage the systemic immune system, as do viruses such as smallpox, measles, and rubella that consistently have a pronounced viremic phase. Additionally, like influenza, SARS-CoV-2 mutates continually into new variants that escape immunity derived from past infections and vaccines. The virus behind COVID-19 can also be easily transmitted asymptomatically, making public containment significantly harder. Moreover, neither infection nor vaccination appears to induce prolonged protection.
“Indeed, for many such respiratory viruses, including SARS-CoV-2, immunity is itself a fluid concept, ranging from complete and durable immunity that fully protects against infections, to immunity that protects against severe disease but does not prevent reinfection and onward transmission,” the authors write.
If mutants continue to emerge or vaccine- and infection-induced immunity continue to wane, SARS-CoV-2 is likely to spread indefinitely, although most likely at a low endemic level.
“This notably has occurred with the 1918 pandemic influenza virus, whose viral descendants still are causing seasonal outbreaks and occasional pandemics 104 years later (pandemic H2N2 in 1957, H3N2 in 1968, and H1N1 in 2009), and which we have been unable, after more than 80 years of trying, to fully control with vaccines,” write Fauci et al.
Still, the authors believe vaccines will play a critical role in controlling SARS-CoV-2 in the future. After more than two years of viral circulation and over a year of vaccines with boosters, there is now, at least, a high degree of background population immunity. Combine that with approved antiviral drugs and monoclonal antibodies to prevent progression of disease and widely available diagnostic tests, and control of COVID-19 without severe disruptions to society is “within our grasp,” the authors say.
It may not look like classical herd immunity but, in this case, that doesn’t seem to make a difference.
“We no longer need the elusive concept of ‘herd immunity’ as an aspirational goal,” the authors conclude.
For the immediate future, the NIAID scientists say more broadly protective vaccines are the goal, including the development of “universal” coronavirus vaccines—or at least universal SARS-CoV-2 vaccines that elicit durable and broadly protective immunity against multiple SARS-CoV-2 variants.