Herd Immunity in Real Life: What Waning Immunity and Policy Shifts Mean for Vaccination and Disease Risk

Latest epidemiological data or published outbreak reports that demonstrate the real-world impact of these policy changes on vaccination rates and disease incidence:

The percentages of serious adverse effects associated with vaccinations for all communicable diseases are extremely low, generally less than 0.002% (2 per 100,000 doses) for serious events. Large-scale pharmacovigilance data from Italy found a serious adverse event reporting rate of 2.2 per 100,000 distributed doses, with fatal outcomes reported in only 0.3% of cases, and no consistent causal association with vaccination established for these fatalities.[1] Anaphylaxis, one of the most serious vaccine-related adverse events, occurs at a rate of approximately 1 per 1,000,000 doses.[2] Other rare serious events include Guillain-Barré syndrome (1 per 1,000,000 for influenza vaccine) and vaccine-associated paralytic poliomyelitis (1 per 1,000,000 for oral polio vaccine).[3][4] 

The United States Centers for Disease Control and Prevention (CDC) and its Advisory Committee on Immunization Practices (ACIP) emphasize that severe adverse reactions are rare and that the risk of serious events is much lower than the risk of complications from the diseases prevented by vaccination.[5] Most adverse events are mild and transient, such as local reactions or fever. 

The table below from the New England Journal of Medicine provides historical context for vaccine safety issues, illustrating the rarity and nature of serious adverse events associated with vaccines. 

References

• Ten Years of Vaccinovigilance in Italy: An Overview of the Pharmacovigilance Data From 2008 to 2017. Moretti F, Gonella L, Gironi S, et al. Scientific Reports. 2020;10(1):14122. doi:10.1038/s41598-020-70996-x.
• Adverse Reactions to Vaccines. Kelso JM, Li JT, Nicklas RA, et al. Annals of Allergy, Asthma & Immunology : Official Publication of the American College of Allergy, Asthma, & Immunology. 2009;103(4 Suppl 2):S1-14. doi:10.1016/s1081-1206(10)60350-x.
• Vaccines and Vaccination. Ada G. The New England Journal of Medicine. 2001;345(14):1042-53. doi:10.1056/NEJMra011223.
• Understanding Vaccine Safety and the Roles of the FDA and the CDC. Meissner HC. The New England Journal of Medicine. 2022;386(17):1638-1645. doi:10.1056/NEJMra2200583.
• General Recommendations on Immunization — Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR. Recommendations and Reports : Morbidity and Mortality Weekly Report. Recommendations and Reports. 2011;60(2):1-64.

Even though the COVID vaccine has waned effectiveness, is it still worth receiving?

Yes, it is still recommended and worthwhile to receive the COVID-19 vaccine. While vaccine effectiveness against SARS-CoV-2 infection and mild symptomatic disease has decreased with the emergence of Omicron and other variants, and protection wanes over time after both primary and booster doses, vaccination—especially with booster doses—continues to provide strong and sustained protection against severe disease, hospitalization, and death.[1][2][3][4][5][6][7][8] Booster doses restore and prolong this protection, particularly for high-risk groups and older adults, and are recommended by the United States Centers for Disease Control and Prevention and its Advisory Committee on Immunization Practices to optimize defense against severe outcomes.[1][6][7][8] 

The main rationale for ongoing COVID-19 vaccination is to prevent serious illness and maintain healthcare system capacity, even as vaccines are less effective at preventing infection with newer variants.[5][4][8] The benefit-risk profile remains strongly favorable, as the risk of severe COVID-19 far exceeds the risk of serious vaccine-related adverse events. Current recommendations include updated bivalent booster doses for all eligible individuals, with specific timing based on age and risk factors.[6] Recent data on long-term protection against severe disease for newer Omicron subvariants may be limited, but available evidence supports continued vaccination to reduce the burden of severe COVID-19. 

References

• Effectiveness of 2, 3, and 4 COVID-19 mRNA Vaccine Doses Among Immunocompetent Adults During Periods When SARS-CoV-2 Omicron BA.1 and BA.2/BA.2.12.1 Sublineages Predominated – VISION Network, 10 States, December 2021-June 2022. Link-Gelles R, Levy ME, Gaglani M, et al. MMWR. Morbidity and Mortality Weekly Report. 2022;71(29):931-939. doi:10.15585/mmwr.mm7129e1.
• Covid-19 Vaccine Effectiveness against the Omicron (B.1.1.529) Variant. Andrews N, Stowe J, Kirsebom F, et al. The New England Journal of Medicine. 2022;386(16):1532-1546. doi:10.1056/NEJMoa2119451.
• Waning of 2-Dose BNT162b2 and mRNA-1273 Vaccine Effectiveness Against Symptomatic SARS-CoV-2 Infection Accounting for Depletion-of-Susceptibles Bias. Andrejko KL, Pry JM, Myers JF, et al. American Journal of Epidemiology. 2023;192(6):895-907. doi:10.1093/aje/kwad017.
• Facing the Omicron Variant-How Well Do Vaccines Protect Against Mild and Severe COVID-19? Third Interim Analysis of a Living Systematic Review. Külper-Schiek W, Piechotta V, Pilic A, et al. Frontiers in Immunology. 2022;13:940562. doi:10.3389/fimmu.2022.940562.
• Covid-19 Vaccines — Immunity, Variants, Boosters. Barouch DH. The New England Journal of Medicine. 2022;387(11):1011-1020. doi:10.1056/NEJMra2206573.
• Interim Recommendations From the Advisory Committee on Immunization Practices for the Use of Bivalent Booster Doses of COVID-19 Vaccines – United States, October 2022. Rosenblum HG, Wallace M, Godfrey M, et al. MMWR. Morbidity and Mortality Weekly Report. 2022;71(45):1436-1441. doi:10.15585/mmwr.mm7145a2.
• Waning of Vaccine Effectiveness Against Moderate and Severe Covid-19 Among Adults in the US From the VISION Network: Test Negative, Case-Control Study. Ferdinands JM, Rao S, Dixon BE, et al. BMJ (Clinical Research Ed.). 2022;379:e072141. doi:10.1136/bmj-2022-072141.
• Effectiveness of the Pre-Omicron COVID-19 Vaccines Against Omicron in Reducing Infection, Hospitalization, Severity, and Mortality Compared to Delta and Other Variants: A Systematic Review. Paul P, El-Naas A, Hamad O, et al. Human Vaccines & Immunotherapeutics. 2023;19(1):2167410. doi:10.1080/21645515.2023.2167410.

Threshold of herd immunity for communicable disease

The threshold percentage required to achieve herd immunity for communicable diseases depends on the basic reproduction number (\(R_0\)) of each pathogen and the effectiveness of the vaccine. For highly contagious diseases such as measles, the required population immunity is ≥95%; this is supported by the World Health Organization and recent analyses showing that measles elimination requires at least 95% coverage with two doses of measles vaccine, especially in children aged 1–9 years.[1][2] For pertussis, the herd immunity threshold is also high, typically ≥92–95%.[3] For influenza, the threshold varies widely depending on the strain and \(R_0\), but is generally 30–40% for seasonal influenza and up to 80–90% for pandemic strains.[4] 

For COVID-19, the herd immunity threshold is variable and depends on the circulating variant. For the ancestral SARS-CoV-2, estimates ranged from 28–70%, but for the Delta variant and more transmissible Omicron subvariants, the threshold may be as high as 80–90%.[5] 

In summary, the herd immunity threshold ranges from 30% to 95% depending on the disease, with the highest requirements for measles and pertussis, and lower thresholds for less transmissible pathogens such as seasonal influenza.[1][5][3][2][4] 

References

• Are the Objectives Proposed by the WHO for Routine Measles Vaccination Coverage and Population Measles Immunity Sufficient to Achieve Measles Elimination From Europe?. Plans-Rubió P. Vaccines. 2020;8(2):E218. doi:10.3390/vaccines8020218.
• Measles 2025. Do LAH, Mulholland K. The New England Journal of Medicine. 2025;. doi:10.1056/NEJMra2504516.
• Vaccination Coverage for Routine Vaccines and Herd Immunity Levels Against Measles and Pertussis in the World in 2019. Plans-Rubió P. Vaccines. 2021;9(3):256. doi:10.3390/vaccines9030256.
• The Vaccination Coverage Required to Establish Herd Immunity Against Influenza Viruses. Plans-Rubió P. Preventive Medicine. 2012;55(1):72-7. doi:10.1016/j.ypmed.2012.02.015.
• Caveats on COVID-19 Herd Immunity Threshold: The Spain Case. García-García D, Morales E, Fonfría ES, Vigo I, Bordehore C. Scientific Reports. 2022;12(1):598. doi:10.1038/s41598-021-04440-z.

Summary of  the latest evidence on how waning immunity and vaccine hesitancy impact the maintenance of herd immunity thresholds over time for these diseases:

Waning immunity and vaccine hesitancy both critically undermine the maintenance of herd immunity thresholds for communicable diseases such as measles, pertussis, influenza, and COVID-19. 

For measles, immunity wanes gradually after vaccination, with seroprevalence studies showing a measurable annual decline in antibody titers, especially after the first dose, though the second dose provides more durable protection. This gradual loss of immunity increases the proportion of susceptible individuals over time, even in highly vaccinated populations, and necessitates maintaining coverage well above the 95% threshold to prevent outbreaks.[1] Vaccine hesitancy has led to persistent suboptimal coverage globally and locally, with recent data showing that no region currently achieves the ≥95% coverage required for measles herd immunity, resulting in recurrent outbreaks and reestablishment of endemic transmission.[2][3] 

For pertussis, waning immunity is even more pronounced, as both vaccine-induced and natural immunity decline rapidly, often within a few years. This means that even with high initial coverage, population immunity can fall below the 92–95% threshold, and outbreaks can occur unless booster doses are administered and coverage is sustained.[3] 

Influenza and COVID-19 are characterized by both waning immunity and antigenic variation. For these pathogens, immunity from vaccination or infection declines within months, and new variants can escape prior immunity. Mathematical models show that when immunity wanes gradually, the amount of vaccine required to maintain herd immunity is substantially higher—up to 150% more for COVID-19—compared to models assuming abrupt loss of immunity.[4] For COVID-19, the herd immunity threshold is dynamic and often unattainable due to rapid waning and variant evolution, so vaccination primarily reduces severe disease rather than achieving elimination.[5][4] 

Vaccine hesitancy further exacerbates these challenges. Declines in routine vaccination rates, driven by hesitancy and policy changes, have resulted in local and national coverage falling below herd immunity thresholds for measles and pertussis in many communities, with a direct correlation between hesitancy and increased susceptibility and outbreak risk.[6][7][8][9][2] 

In summary, waning immunity and vaccine hesitancy both lower population immunity below critical herd immunity thresholds, increasing the risk of outbreaks and reestablishment of endemic transmission for highly contagious diseases.[6][7][9][2][4][1][3] 

References

• Quantification of Waning Immunity After Measles Vaccination-Evidence From a Seroprevalence Study. Zibolenová J, Hudečková H, Chladná Z, et al. American Journal of Epidemiology. 2023;192(8):1379-1385. doi:10.1093/aje/kwad065.
• Measles 2025. Do LAH, Mulholland K. The New England Journal of Medicine. 2025;. doi:10.1056/NEJMra2504516.
• Vaccination Coverage for Routine Vaccines and Herd Immunity Levels Against Measles and Pertussis in the World in 2019. Plans-Rubió P. Vaccines. 2021;9(3):256. doi:10.3390/vaccines9030256.
• Extending Susceptible-Infectious-Recovered-Susceptible Epidemics to Allow for Gradual Waning of Immunity. El Khalifi M, Britton T. Journal of the Royal Society, Interface. 2023;20(206):20230042. doi:10.1098/rsif.2023.0042.
• Herd Immunity, Vaccination and Moral Obligation. Bullen M, Heriot GS, Jamrozik E. Journal of Medical Ethics. 2023;49(9):636-641. doi:10.1136/jme-2022-108485.
• Estimating the Number of US Children Susceptible to Measles Resulting From COVID-19-related Vaccination Coverage Declines. Gambrell A, Sundaram M, Bednarczyk RA. Vaccine. 2022;40(32):4574-4579. doi:10.1016/j.vaccine.2022.06.033.
• Evidence on Trends in Uptake of Childhood Vaccines and Association With COVID-19 Vaccination Rates. Moghtaderi A, Callaghan T, Luo Q, et al. Vaccine. 2025;45:126631. doi:10.1016/j.vaccine.2024.126631.
• Mitigating Vaccine Hesitancy and Building Trust to Prevent Future Measles Outbreaks in England. Thompson S, Meyer JC, Burnett RJ, Campbell SM. Vaccines. 2023;11(2):288. doi:10.3390/vaccines11020288.
• Modeling Reemergence of Vaccine-Eliminated Infectious Diseases Under Declining Vaccination in the US. Kiang MV, Bubar KM, Maldonado Y, Hotez PJ, Lo NC. JAMA. 2025;333(24):2176-2187. doi:10.1001/jama.2025.6495.

Waning immunity and vaccine hesitancy both critically undermine the maintenance of herd immunity thresholds for communicable diseases such as measles, pertussis, influenza, and COVID-19. 

For measles, immunity wanes gradually after vaccination, with seroprevalence studies showing a measurable annual decline in antibody titers, especially after the first dose, though the second dose provides more durable protection. This gradual loss of immunity increases the proportion of susceptible individuals over time, even in highly vaccinated populations, and necessitates maintaining coverage well above the 95% threshold to prevent outbreaks.[1] Vaccine hesitancy has led to persistent suboptimal coverage globally and locally, with recent data showing that no region currently achieves the ≥95% coverage required for measles herd immunity, resulting in recurrent outbreaks and reestablishment of endemic transmission.[2][3] 

For pertussis, waning immunity is even more pronounced, as both vaccine-induced and natural immunity decline rapidly, often within a few years. This means that even with high initial coverage, population immunity can fall below the 92–95% threshold, and outbreaks can occur unless booster doses are administered and coverage is sustained.[3] 

Influenza and COVID-19 are characterized by both waning immunity and antigenic variation. For these pathogens, immunity from vaccination or infection declines within months, and new variants can escape prior immunity. Mathematical models show that when immunity wanes gradually, the amount of vaccine required to maintain herd immunity is substantially higher—up to 150% more for COVID-19—compared to models assuming abrupt loss of immunity.[4] For COVID-19, the herd immunity threshold is dynamic and often unattainable due to rapid waning and variant evolution, so vaccination primarily reduces severe disease rather than achieving elimination.[5][4] 

Vaccine hesitancy further exacerbates these challenges. Declines in routine vaccination rates, driven by hesitancy and policy changes, have resulted in local and national coverage falling below herd immunity thresholds for measles and pertussis in many communities, with a direct correlation between hesitancy and increased susceptibility and outbreak risk.[6][7][8][9][2] 

In summary, waning immunity and vaccine hesitancy both lower population immunity below critical herd immunity thresholds, increasing the risk of outbreaks and reestablishment of endemic transmission for highly contagious diseases.[6][7][9][2][4][1][3] 

References

• Quantification of Waning Immunity After Measles Vaccination-Evidence From a Seroprevalence Study. Zibolenová J, Hudečková H, Chladná Z, et al. American Journal of Epidemiology. 2023;192(8):1379-1385. doi:10.1093/aje/kwad065.
• Measles 2025. Do LAH, Mulholland K. The New England Journal of Medicine. 2025;. doi:10.1056/NEJMra2504516.
• Vaccination Coverage for Routine Vaccines and Herd Immunity Levels Against Measles and Pertussis in the World in 2019. Plans-Rubió P. Vaccines. 2021;9(3):256. doi:10.3390/vaccines9030256.
• Extending Susceptible-Infectious-Recovered-Susceptible Epidemics to Allow for Gradual Waning of Immunity. El Khalifi M, Britton T. Journal of the Royal Society, Interface. 2023;20(206):20230042. doi:10.1098/rsif.2023.0042.
• Herd Immunity, Vaccination and Moral Obligation. Bullen M, Heriot GS, Jamrozik E. Journal of Medical Ethics. 2023;49(9):636-641. doi:10.1136/jme-2022-108485.
• Estimating the Number of US Children Susceptible to Measles Resulting From COVID-19-related Vaccination Coverage Declines. Gambrell A, Sundaram M, Bednarczyk RA. Vaccine. 2022;40(32):4574-4579. doi:10.1016/j.vaccine.2022.06.033.
• Evidence on Trends in Uptake of Childhood Vaccines and Association With COVID-19 Vaccination Rates. Moghtaderi A, Callaghan T, Luo Q, et al. Vaccine. 2025;45:126631. doi:10.1016/j.vaccine.2024.126631.
• Mitigating Vaccine Hesitancy and Building Trust to Prevent Future Measles Outbreaks in England. Thompson S, Meyer JC, Burnett RJ, Campbell SM. Vaccines. 2023;11(2):288. doi:10.3390/vaccines11020288.
• Modeling Reemergence of Vaccine-Eliminated Infectious Diseases Under Declining Vaccination in the US. Kiang MV, Bubar KM, Maldonado Y, Hotez PJ, Lo NC. JAMA. 2025;333(24):2176-2187. doi:10.1001/jama.2025.6495.