Herd immunity to endemic diseases: Historical concepts and implications for public health policy

BACKGROUND

"Herd immunity" became a contested term during the COVID-19 pandemic. Although the term "herd immunity" is often used to refer to thresholds at which some diseases can be eliminated (e.g., due to mass vaccination), the term has multiple referents. Different concepts of herd immunity have been relevant throughout the history of immunology and infectious disease epidemiology. For some diseases, herd immunity plays a role in the development of an endemic equilibrium, rather than elimination via threshold effects.

METHODS

We reviewed academic literature from 1920 to 2022, using historical and philosophical analysis to identify and develop relevant concepts of herd immunity.

RESULTS

This paper analyses the ambiguity surrounding the concept of herd immunity during the pandemic. We argue for the need to recapture a long-standing interpretation of this concept as one of the factors that leads to a dynamic endemic equilibrium between a host population and a mutating respiratory pathogen.

CONCLUSIONS

Informed by the history of infectious disease epidemiology, we argue that understanding the concept in this way will help us manage both SARS-CoV-2 and hundreds of other seasonal respiratory pathogens with which we live but which have been disrupted due to sustained public health measures/non-pharmaceutical interventions targeting SARS-CoV-2.

Vaccine-enhanced disease: case studies and ethical implications for research and public health

Vaccination is a cornerstone of global public health. Although licensed vaccines are generally extremely safe, both experimental and licensed vaccines are sometimes associated with rare serious adverse events. Vaccine-enhanced disease (VED) is a type of adverse event in which disease severity is increased when a person who has received the vaccine is later infected with the relevant pathogen. VED can occur during research with experimental vaccines and/or after vaccine licensure, sometimes months or years after a person receives a vaccine. Both research ethics and public health policy should therefore address the potential for disease enhancement. Significant VED has occurred in humans with vaccines for four pathogens: measles virus, respiratory syncytial virus, Staphylococcus aureus, and dengue virus; it has also occurred in veterinary research and in animal studies of human coronavirus vaccines. Some of the immunological mechanisms involved are now well-described, but VED overall remains difficult to predict with certainty, including during public health implementation of novel vaccines. This paper summarises the four known cases in humans and explores key ethical implications. Although rare, VED has important ethical implications because it can cause serious harm, including death, and such harms can undermine vaccine confidence more generally – leading to larger public health problems. The possibility of VED remains an important challenge for current and future vaccine development and deployment. We conclude this paper by summarising approaches to the reduction of risks and uncertainties related to VED, and the promotion of public trust in vaccines.

International Liaison Committee on Resuscitation: COVID-19 consensus on science, treatment recommendations and task force insights

Consensus on Science and Treatment recommendations aim to balance the benefits of early resuscitation with the potential for harm to care providers during the COVID-19 pandemic. Chest compressions and cardiopulmonary resuscitation have the potential to generate aerosols. During the current COVID-19 pandemic lay rescuers should consider compressions and public-access defibrillation. Lay rescuers who are willing, trained and able to do so, should consider providing rescue breaths to infants and children in addition to chest compressions. Healthcare professionals should use personal protective equipment for aerosol generating procedures during resuscitation and may consider defibrillation before donning personal protective equipment for aerosol generating procedures.

Echocardiography has minimal yield and may not be warranted in Staphylococcus aureus bacteremia without clinical risk factors for endocarditis

Current guidelines recommend performing echocardiography in all patients with Staphylococcus aureus bacteremia (SAB), but patients at very low risk of endocarditis may not benefit from this investigation. This study seeks to identify patients at very low risk of endocarditis. A retrospective single-center consecutive case series of patients with SAB was examined. Microbiological and echocardiographic data were used to identify patients with community onset, prolonged bacteremia, and intracardiac prosthetic devices. The diagnostic performance of these criteria for endocarditis as measured against transesophageal echocardiography (TEE) was calculated. 593 episodes of SAB were examined over a period of 6 years. 10 % were excluded from analysis due to death or discharge less than 48 h after the first positive blood culture or no admission to hospital, leaving 532 episodes for analysis. 64 % of the included episodes were investigated with echocardiography: 39 % with TEE and 26 % with transthoracic echocardiography (TTE) only. 16 % of the episodes investigated with echocardiography were demonstrated to have endocarditis. The rate of endocarditis was higher for episodes undergoing TEE (24 %) than TTE only (5 %). There were no instances of endocarditis amongst the 23 episodes investigated with TEE where none of the three risk factors were present. This group represented 57 % of the nosocomial (non-community-onset) episodes investigated with TEE. Patients with none of the three criteria examined in this study have a very low rate of endocarditis and may fall below the test threshold for echocardiography.