The CDC estimates that in the past 20 years, vaccines have saved more that 21 million lives. There are many different types of vaccines that provide protection against infectious diseases. Some examples of diseases for which vaccines elicit protective immunity include diphtheria, whooping cough, tetanus, measles, mumps, rubella, chicken pox and hepatitis. Protective immunity means that vaccinated individuals will not get the disease they are vaccinated against.
Naturally Acquired Immunity
Natural immunity occurs after becoming infected with a pathogen such as SARS-CoV-2, the virus that causes COVID-19, naturally, which, in the case of COVID-19, means contracting the virus from secretions or airborne droplets from an infected person. Indeed, most people survive a natural infection with SARS-CoV-2 because they develop neutralizing antibodies and T cells that kill virally infected cells. During a natural infection with SARS-CoV-2, the virus replicates in our cells producing millions of viral proteins, which alerts the immune system that a foreign invader is present. Since the immune system does not know which viral proteins are targets for neutralization, it makes antibodies against all viral proteins. As such, everyone infected with COVID-19 makes antibodies to all viral proteins. But, not all antibodies are created equal. Only neutralizing antibodies are effective in controlling spread of the virus, both within the body and from person-to-person. However, research has shown that up to one-third of individuals with COVID-19 do not generate detectable levels of neutralizing antibodies after natural infection.
Clearly, natural immunity occurs on the virus’ terms. We have no control over the dose of virus we are infected/inoculated with in airborne droplets or secretions, or how the virus co-opts cellular machinery once inside our cells, or the types of cells it infects, and certainly no control over our genetics which dictate how sick we might get from the virus.
Asymptomatic vs. Symptomatic infections
According to the CDC, the younger the individual, the less severe COVID-19 symptoms. Many younger individuals who have spread COVID-19 to others resulting in “super-spreader” events never even knew they were infected; they were asymptomatic. Scientists are still trying to understand why some people have different severities of COVID-19 illness, from asymptomatic, to mild cold, to severe cold and u-like symptoms, to hospitalization in the intensive care unit.
Some have hypothesized that the Coronavirus family of viruses employs an evolutionary strategy designed to not cause symptoms or elicit high levels of neutralizing antibodies upon natural infection because strong immunity and symptomatic infection would limit spread of virus. Infection without strong immunity results in a carrier state in which the virus can be transmitted to non-immune individuals and eventually may even re-infect those who did not mount strong immunity against the virus the first time. Because natural infection is on the virus’ terms, we have little control over how we respond to the infection.
Vaccine Immunity vs. Naturally Acquired Immunity
In contrast to a natural infection, vaccine immunity is induced on our terms. Modern vaccines can be developed when scientists identify the “achilles heel” of the virus that is crucial for infection. Only three months after identification of SARS-CoV-2, scientists identified a specific region on the viral surface that allows the virus to bind to its cellular receptor. They proved this region was required for infection by generating antibodies against it in mice and then showing that those mouse antibodies block SARS-CoV-2 from infecting cells. As further proof, human antibodies from recovered COVID-19 patients that bind to this region of the virus also blocked infection. This finding was very important for companies developing COVID-19 vaccines, because it provided a target for their vaccines. It is the goal of all COVID-19 vaccines to induce neutralizing antibodies which protect against re-infection. This is a foundational concept in immunology; there is no controversy. Vaccines may also induce T cells that can kill virally infected cells, but primarily vaccines induce neutralizing antibodies that shield recipients from infection and re-infection.
Unlike natural immunity, vaccines cut right to the chase. COVID-19 vaccines are composed primarily of viral proteins that elicit neutralizing antibodies (and anti-viral killer T cells). Vaccines focus the body’s immune response so it doesn’t waste time and energy making antibodies to non-neutralizing viral proteins. The immune response makes antibodies only against the region of the virus that binds to the cellular receptor having the effect of physically blocking the virus from binding to a cell. Another advantage of vaccines compared to natural infection is that vaccines can include components that can increase the longevity of immunity. While neutralizing antibodies generated in response to a natural infection may wane after a few months, vaccine formulations extend the longevity of protective neutralizing antibodies.
At this point in time, we do not know how long vaccine immunity will last, but new tests are on the horizon that can measure levels of not just any COVID-19 antibody, but specific neutralizing antibodies. Given the world-wide economic disruption, morbidity and mortality of COVID-19, it might be advisable to monitor levels of neutralizing antibodies such that one might get a vaccine booster when neutralizing antibodies decline to non-protective levels.