Coronavirus (COVID-19) Frequently Asked Questions 2459

Testing and Laboratory
What is the evidence for covid-19 immunity in Trinity County?

The question of immunity in the context of COVID-19 is a complex one no matter the county. It is difficult to state with any degree of certainty that there is evidence of immunity in Trinity County. Below is background information on individual and community immunity principles and a few highlighted findings of some of the studies that have been conducted to date.

The human immune system is a highly integrated physiologic defense system against a variety of threats, such as viruses, including novel (new) viruses not previously seen in humans.
Collectively these studies seem to indicate that there may be at least some short term protection (potentially up to 3 months) but the questions continue around variations in individuals and what constitutes protective immunity over time, sufficient to create community immunity.

Below is a very brief overview of the immune system in humans and what emerging science is telling us about the immune response specific to COVID-19. I am also attaching links to reference material.

The human immune system consists of two parts: The innate immune system and the adaptive immune system.
Innate immunity is immediate and general or nonspecific, meaning that anything identified as foreign or "non-self" are targets for response including bacteria and viruses. The innate immune system is the first line of defense and consists of physical, chemical, and cellular defenses against pathogens. Skin, as a barrier first line defense, as well as proteins in chemical form such as the complement system are examples of components of innate immunity.
Adaptive immunity, also called acquired immunity, is specific to a particular pathogen (such as viruses) and can take weeks or months to develop. The latter system consists of three parts: antibodies, B cells and T cells. Together they recognize and fight off the invader and can store a memory of it in case of future infection (vaccines similarly work by creating a fake “memory”). Some of these antibodies, known as neutralizing antibodies, can bind to a specific part of a pathogen and deactivate it.

Some definitions that will be helpful:
Acquired immunity - established at the level of the individual, either through natural infection with a pathogen or through immunization with a vaccine.
Community Immunity or Herd Immunity - stems from the effects of individual immunity scaled to the level of the population. It refers to the indirect protection from infection conferred to susceptible individuals when a sufficiently large proportion of immune individuals exist in a population. This population-level effect is often considered in the context of vaccination programs, which aim to establish community immunity so that those who cannot be vaccinated, including the very young and immunocompromised, are still protected against disease.
Ro  -  is an estimate of contagiousness that is a function of human behavior and biological characteristics of pathogens. In simple terms it is the number of cases, on average, an infected person will cause during their infectious period.
SARS-CoV-2 – The virus that causes COVID-19.
Seroprevalence - The overall occurrence of a disease or condition, in this case SARS- CoV-2, within a defined population at one time and measured through serology tests.

Numerous studies continue around the question of immunity and COVID-19. Many of these studies show that people who have recovered from infection with SARS-CoV-2 have some antibodies to the virus; however, it is unknown whether all infected patients mount a protective immune response and how long any protective effect will last. Nevertheless, the short-term risk of reinfection (i.e. within the first few months after initial infection) appears low.

“Among a convenience sample of 382 young adult U.S. service members aboard an aircraft carrier experiencing a COVID-19 outbreak, 60% had reactive antibodies, and 59% of those also had neutralizing antibodies at the time of specimen collection. One fifth of infected participants reported no symptoms. Preventive measures, such as using face coverings and observing social distancing, reduced risk for infection.” “…The presence of neutralizing antibodies among the majority is a promising indicator of at least short-term immunity.”

Many aspects of the immune response to SARS-CoV-2 infection remain unknown. Understanding rates of seroconversion among asymptomatic persons, the duration of detectable circulating antibodies in relation to illness severity, and the potential impact of host factors (e.g., age and underlying medical conditions) on seroconversion are essential for interpreting SARS-CoV-2 immunity over time. It is also unknown whether these antibodies, (identified through serology testing) confer immunity, a critical factor in understanding the implications of seroprevalence estimates.

In a sufficiently immune population, community immunity provides indirect protection to susceptible individuals by minimizing the probability of an effective contact between a susceptible individual and an infected host. In its simplest form, community immunity will begin to take effect when a population reaches the community immunity threshold, namely when the proportion of individuals who are immune to the pathogen crosses 1 – 1/R0. At this point, sustained transmission cannot occur, so the outbreak will decline. However, in real-world populations, the situation is often much more complex. Epidemiological and immunological factors, such as population structure, variation in transmission dynamics between populations, and waning immunity, will lead to variation in the extent of indirect protection conferred by community immunity. Consequently, these aspects must be taken into account when discussing the establishment of community immunity within populations. There are two possible approaches to build widespread SARS-CoV-2 immunity: (1) a mass vaccination campaign, which requires the development of an effective and safe vaccine, or (2) natural immunization of populations with wild virus over time. However, the consequences of number (2) are serious and far-reaching—a large fraction of the human population would need to become infected with the virus, and millions would succumb to it. Thus, in the absence of a vaccination program, an emphasis should be placed on policies that protect the most vulnerable groups in our populations.