Question

Consider the following hypothetical diseases, and answer part A, B, and C below:
Disease X is very deadly with a mortality rate >30%. However, it is only spread through bodily fluids and so is difficult to transmit between people.
Disease Y produces mild symptoms, and has a mortality rate <1%. However, it can be spread through the air and remains infectious on unwashed surfaces. Because of this, it is extremely transmissible.

A) Which disease will require a higher vaccination rate to prevent its spread? (0.5 points)
B) Explain what characteristics of the disease you chose in Part A make it a good candidate for high vaccination rates. (0.5 points)
C) Explain how a sufficiently high vaccination rate leads to Herd Immunity at a population level. (1 point)

Answer 1

a. Vaccination is development of immunity against the pathogen via injection of live attenuated, dead pathogen or component of the pathogen. Vaccination is required to prevent transmission of the disease. They may or may not ensure that the disease is treated after the infection has set in. Rates of vaccination depend on the Ro number. Ro is the reproduction number and will measure the actual transmission of the pathogen in the population. Higher the Ro, greater will be its transmission rate.

The disease Y has a higher transmission rate than disease X, although it is milder than disease X. As vaccines only prevent spread of infection, they may not alter the mortality rate after infection. They just prevent the development of the disease by preventing infection itself. Hence, vaccination will not reduce the mortality rates of disease X as these individuals may not be infected in first place. Thus, the disease Y will require higher vaccination rate than disease X due to its higher Ro. This is because disease Y is an airborne infection and thus, infected larger number of population. Disease Y although more lethal, will infect lesser number of people. Hence, disease Y has a lower Ro value and hence, requires lesser vaccination rate.

Herd immunity is the automatic immunity attained in the rest of the population to a disease when most of the individuals become immune to the disease either by vaccination or being infected. Ro also determines the herd immunity. If Ro is lower, it requires lower immunity to obtain herd immunity. Conversely, if Ro is higher, more numbers need to be vaccinated to obtain Herd immunity.

Right choice: Disease Y.

b. Characteristics of Disease Y for higher vaccination rates:

1. The disease Y has a faster rate of transmission as it is airborne. Thus, one person can infect more individuals in the population by directly spreading the infection via droplets released upon sneezing or coughing. Further, people can get infected by touching surfaces that harbor the pathogen. This indicates a very high Ro or reproduction number. Higher the Ro, higher is the need for vaccination to inhibit spread of the disease.

2. Higher the rate of transmission, higher the number of people that need to be vaccinated in order to obtain herd immunity. Hence, disease Y requires higher vaccination rate.

c. Vaccines will protect the individuals from infectious agent by producing antibodies that neutralize the pathogen before it can cause the disease. Herd immunity is the immunity in the non-vaccinated population due to decreased transmission of the pathogen as a result of vaccination of major individuals in the population. When the vaccination rate is higher, there will be more number of people who will become immune to the disease. Hence, they will not be infected and will not transmit the disease to neighboring individual in the population. Thus, there will be lesser transmission of the disease to other uninfected individuals. This is because the pathogen cannot find sufficient hosts to infect. Hence, disease transmission will be reduced and then completely eliminated. This leads to Herd immunity in uninfected individuals and the population will become immune to the pathogen in due course.