The coronavirus disease 2019 (COVID-19) is one of the biggest public health threats in the 21st century. Nearly every country in the world has been affected by COVID-19. The magnitude of the problem, with over 179 million confirmed cases and 3.8 million deaths worldwide, has driven researchers to search for vaccines to combat the disease. The discovery and development of a new vaccine, from the initial stage to the vaccine finally reaching the patients, usually take many years. However, given the urgency of the situation, many clinical trials on the COVID-19 vaccines have been conducted at extraordinary speed, whereas several vaccines against SARS-CoV-2 are being administered worldwide. This article gives an overview of the different types of COVID-19 vaccines, with a focus on those with promising results and are commonly used worldwide. It also gives an overview of herd immunity and discusses the challenges in achieving herd immunity through the global vaccination campaigns. Last but not least, some strategies that may be used to address these challenges are discussed.
The conventional susceptible-infectious-recovered (SIR) model tends to magnify the transmission dynamics of infectious diseases, and thus the estimated total infections and immunized population may be higher than the threshold required for infection control and eradication. The study developed a new SIR framework that allows the transmission rate of infectious diseases to decline along with the reduced risk of contact infection to overcome the limitations of the conventional SIR model. Two new SIR models were formulated to mimic the declining transmission rate of infectious diseases at different stages of transmission. Model A utilized the declining transmission rate along with the reduced risk of contact infection following infection, while Model B incorporated the declining transmission rate following recovery. Both new models and the conventional SIR model were then used to simulate an infectious disease with a basic reproduction number (r0) of 3.0 and a herd immunity threshold (HIT) of 0.667 with and without vaccination. Outcomes of simulations were assessed at the time when the total immunized population reached the level predicted by the HIT, and at the end of simulations. Further, all three models were used to simulate the transmission dynamics of seasonal influenza in the United States and disease burdens were projected and compared with estimates from the Centers for Disease Control and Prevention. For the simulated infectious disease, in the initial phase of the outbreak, all three models performed expectedly when the sizes of infectious and recovered populations were relatively small. As the infectious population increased, the conventional SIR model appeared to overestimate the infections even when the HIT was achieved in all scenarios with and without vaccination. For the same scenario, Model A appeared to attain the level predicted by the HIT and in comparison, Model B projected the infectious disease to be controlled at the level predicted by the HIT only at high vaccination rates. For infectious diseases with high r0, and at low vaccination rates, the level at which the infectious disease was controlled cannot be accurately predicted by the current theorem. Transmission dynamics of infectious diseases with herd immunity can be accurately modelled by allowing the transmission rate of infectious diseases to decline along with the reduction of contact infection risk after recovery or vaccination. Model B provides a credible framework for modelling infectious diseases with herd immunity in a randomly mixed population.
Pneumococcal disease causes large morbidity, mortality and health care utilization and medical and non-medical costs, which can all be reduced by effective infant universal routine immunization programs with pneumococcal conjugate vaccines (PCV). We evaluated the clinical and economic benefits of such programs with either 10- or 13-valent PCVs in Malaysia and Hong Kong by using an age-stratified Markov cohort model with many country-specific inputs. The incremental cost per quality-adjusted life year (QALY) was calculated to compare PCV10 or PCV13 against no vaccination and PCV13 against PCV10 over a 10-year birth cohort's vaccination. Both payer and societal perspectives were used. PCV13 had better public health and economic outcomes than a PCV10 program across all scenarios considered. For example, in the base case scenario in Malaysia, PCV13 would reduce more cases of IPD (+2,296), pneumonia (+705,281), and acute otitis media (+376,967) and save more lives (+6,122) than PCV10. Similarly, in Hong Kong, PCV13 would reduce more cases of IPD cases (+529), pneumonia (+172,185), and acute otitis media (+37,727) and save more lives (+2,688) than PCV10. During the same time horizon, PCV13 would gain over 74,000 and 21,600 additional QALYs than PCV10 in Malaysia and Hong Kong, respectively. PCV13 would be cost saving when compared against similar program with PCV10, under both payer and societal perspective in both countries. PCV13 remained a better choice over PCV10 in multiple sensitivity, scenario, and probabilistic analyses. PCV13s broader serotype coverage in its formulation and herd effect compared against PCV10 were important drivers of differences in outcomes.