What is Herd Immunity?
Herd Immunity is an epidemiological concept where most of the population of a region is immune to a contagious disease. It prevents people from getting sick or become carriers of the disease even if they are exposed to infected people.
Herd immunity or community immunity is built up through mass vaccination or a population contracting a disease previously and naturally building immunity against it. This provides a measure of protection to the people who are not vaccinated and vulnerable individuals in a population who have an immune deficiency or suppressed immune systems due to medical conditions such as HIV and people undergoing chemotherapy.
How Does Herd Immunity Work?
Let’s suppose a community achieved herd immunity after 80% of its population became immune to a contagious disease through naturally built immunity and mass vaccination. The individuals with immunity act as a natural buffer to those individuals that are infected. As a result, those with immunity do not become infected and do not transmit the virus to others. This way, the spread of a virus is slowed and eventually stopped altogether. An example of herd immunity at work is the eradication of smallpox.
Depending on the infection, approximately 70%-90% of a population has to be immune naturally or through vaccination to achieve herd immunity. This is called the herd immunity threshold. For example, 92% of the population had to become immune to mumps to reach herd immunity and stop the spread of infection. Measles, polio, mumps, etc. were once very contagious diseases in the US. After extensive vaccination, herd immunity against these diseases was achieved, and they were eradicated. Sporadic outbreaks of infections are seen when vaccine coverage lessens. A recent example of this phenomenon is the 2019 measles outbreak, which happened in Disneyland.
Importance of Herd Immunity
Fifty million people lost lives during the 1918 flu pandemic. During the polio pandemic in 1956 in Argentina, thousands died, and more were left with permanent neurological impact. Once vaccines were discovered, we have developed herd immunity against these communicable diseases through mandatory mass immunizations of the population.
Consider a scenario where a whole city has never come in contact with a virus, e.g., mumps. The inhabitants have no natural immunity. If a sick person with a mumps infection arrives in this city, the results could be disastrous. Mumps, like many other bacterial and viral infections, spread through contact from person to person. Mumps has a basic reproductive rate (R0) of 10 – 12 people. This means that one ill person, on average, will infect 12 new people with the virus. Without resistance, the disease will quickly disseminate throughout the community, spreading from person to person until most are infected. The more infectious a virus, the higher the herd immunity threshold.
On the other hand, consider the same city where 92% of the population is immune from vaccination. When an infected person arrives in this city, the infection will see much more resistance. When most people are immune, they do not get infected or transmit the infection. The chain of infection is broken, and the spread is easily contained.
How Vaccination Is Important for Herd Immunity
Vaccines are substances made from the causative agents (bacteria or virus) of a disease. Vaccines essentially trick your body into thinking it is infected with a viral or bacterial disease and stimulates the body to develop antibodies to counter the threat. Once these antibodies are developed, they are ready to neutralize a real infection caused when the body encounters the pathological agent. Now you are considered immune. Once you are immune, you will not be contagious and transmit the disease any further. Thus herd immunity is achieved with more individuals immune. Vaccination is the safest method to ensure immunity against communicable diseases.
Herd immunity in one age group ensures herd immunity for other age groups. Vaccinating adults against whooping cough reduces occurrence in infants who are most vulnerable to the infection. Vaccination is important for family members and people who come in contact with infants. Similarly, vaccinating children against pneumonia and rotavirus reduce hospitalizations for older adults who are at risk of contracting them. Influenza is more severe in older adults than in younger children. But flu vaccines are not as effective for them due to a weaker immune system because of age. Vaccinating school-age children for seasonal flu to prevent the spread of the virus is more effective than vaccinating the elderly.
Risks to Herd Immunity
In some cases, long term immunity cannot be achieved through vaccination. Some viruses and bacteria mutate frequently and the evolved contagion escapes triggering our natural immune response. The body has to develop a new antibody to counter the mutated contagion. Immunity for such illnesses (e.g., seasonal flu) does not last long as the virus transforms often. Vaccines have to be administered regularly so the body can produce a new antibody to counter it.
For those that travel internationally, the CDC provides a complete list of vaccinations required for travel. There are infections that have not been eradicated worldwide, and as a result, there can be spontaneous outbreaks. HHS recommends that people get vaccinated 4 to 6 weeks before traveling. Viruses can make a comeback when people travel to an affected region and then return to their own country. Vaccination not only protects you from infection, but it also protects the community helping to prevent a virus from being transmitted.
COVID-19 and Herd Immunity
As with any other infectious disease, herd immunity can be achieved through vaccination and infection. Infection then leads to natural immunity. The basic reproductive rate (R0) of coronavirus is 3, which is much lower than viruses for mumps, measles, etc. Even with this R0, the herd immunity threshold for coronavirus is estimated to be around 70% of the population. The best estimates to date put the coronavirus fatality rate at about 0.5% to 1.0%. This would result in a fatality rate of 0.35% – 0.7% of the population. Until a vaccine is developed for Covid-19, the most effective way to contain the spread of COVID-19 is by practicing physical distancing and good hygiene like frequently washing hands. It is imperative to keep safe those at higher risk of being infected due to old age, compromised immune system, chemotherapy treatment, chronic lung diseases, heart conditions, chronic kidney illness and undergoing dialysis, diabetes, and obesity.