The SARS-CoV-2, or COVID-19, pandemic is at the forefront of most popular media. All the information about herd immunity, vaccines, and reinfections can be confusing, especially without prior medical knowledge. There is a lot about the virus that we don’t know, which makes it hard to predict what strategies will work best to mitigate it. Not all infectious diseases impact your immune system the same way. So where does COVID-19 fit in? 

When your body is first introduced to a pathogen, or a disease causing agent like a virus or bacteria, it activates a primary immune response since the pathogen is completely foreign to your body. Because the pathogen is new to you, your body will take longer to get rid of it. This is when you can get common symptoms like sore throat and fever, which are signs your body is trying to fight off the pathogen. Once your body is introduced to a pathogen for the first time, it has a way to make itself less vulnerable to secondary infections. It does this by creating long lasting proteins and cells, known as antibodies and B-cells that are made to specially recognize specific pathogens. If the pathogens ever get in your body again, the antibodies and B-cells will already be there to help eliminate the pathogen before you get sick, unlike the very first time it entered your body. Antibodies and B-cells can last in your system for very different times depending on the pathogen present. This is something scientists are currently trying to figure out about COVID-19, as it is unclear whether or not being infected once will protect you from getting infected again. Currently, we can better our understanding of what mechanism COVID-19 can trigger by looking at other well known infectious diseases, which can fall anywhere on an immunity spectrum. 

One great example of one end of the immunity spectrum is chicken pox, caused by the Varicella-zoster virus. As you probably have heard before, people that get chicken pox are protected from ever getting it again. This is because getting infected once with this particular virus usually leads to lifelong immunity. The immunity is lifelong because the antibodies and B-cells for the Varicella-zoster virus tend to stay in your body for a very long time, protecting you from reinfections from childhood through adulthood. This is also why you only need two vaccine injections for chicken pox in order to be protected. Unfortunately, it is unlikely that COVID-19 will follow the example of the chicken pox. As it is still such a new virus, it is hard to pinpoint how long the antibodies fighting against it will stay in the system. Early reports in places like South Korea show that antibody levels drop off after a few weeks post-infection. 

The next example, which is a little lower on the immunity spectrum, is whooping cough. Whooping cough causes your body to produce antibodies that stay in your system for 4-20 years. While this is a long time, it is not long enough to totally protect you from getting sick, which is why whooping cough requires booster shots, or secondary vaccinations that help keep your immunity high enough to prevent reinfection. COVID-19 might end up with a similar situation, where you can get an initial vaccine but need boosters to keep your immunity up. 

H1N1, or swine flu, is another good example. This is the influenza strain that caused a pandemic in 2009. Studies around H1N1 showed that the immunity can last in the body anywhere between 2-10 years. While in the past it was a very novel infection, it now circulates alongside other flu strains every winter. This is related to the reason you need a new flu shot every year. With so many variations between and changes within individual flu strains, the strain that is most common changes year to year, meaning scientists must predict which strain will be the most prevalent every winter and tailor the yearly vaccine accordingly. Some people believe COVID-19 will behave similarly, causing a dangerous pandemic at first but then assimilating into society as a more common, less dangerous, cold virus. 

When trying to figure out how COVID-19 affects our immunity, it may be most helpful to compare it to the original SARS virus. SARS stands for Severe Acute Respiratory Syndrome and originated in 2002 after the emergence of a specific strain of coronavirus. In 2003, after the virus infected around 8000 people, studies showed that the antibodies against the virus peaked around 4 months after infection and lasted around 2 years. Early studies with COVID-19 have shown that it causes production of short lived, neutralizing antibodies that stay in the body for a few weeks. While a few weeks doesn’t sound as promising as 2 years, the genetic similarity between SARS and COVID-19 may be a good sign that this immunity path is one that we could see for COVID-19. 

Another infectious disease that is fairly similar to COVID-19 is the common cold, which can be caused by four other similar strains of coronavirus. The cold causes immunity that lasts usually a few months and is gone within a year, meaning you are vulnerable to reinfection every year. The good news about colds is they usually don’t cause very serious illness and don’t infect as many people after a certain number of people get infected. 

The other end of the immunity spectrum is HIV, or Human Immunodeficiency Virus. This is the virus that causes AIDS, or Acquired ImmunoDeficiency Syndrome. What makes HIV so dangerous is how it can evade our bodies immune system. HIV is constantly mutating and changing its surface proteins when it is replicating inside the human body, making it very difficult for the body to create the proper antibodies to get rid of the virus. The antibodies for HIV also don’t stay in the body for a very long time. The good news concerning COVID-19 is that it doesn’t seem to mutate as frequently as HIV, meaning it is not as much of a moving target for our bodies to fight off. 

Where COVID-19 falls on the immunity spectrum will change how herd immunity for the virus can be achieved. Herd immunity is a term referring to when enough people in a population have immunity to a pathogen, making it difficult for the pathogen to spread effectively anymore. This can protect people who are immunocompromised or haven’t been infected previously. Immunity can be obtained either by infection or vaccination. The problem with achieving herd immunity for COVID-19 centers around the fact that not much is known still about how the virus functions. There is evidence to suggest that people may develop varying antibody protection based on if they were asymptomatic or symptomatic. Research has shown that asymptomatic individuals may have a weaker immune response to COVID-19 and, thus, lower antibody levels. It is unsure at the moment if these lower levels will make individuals more susceptible to reinfection. In terms of herd immunity, these findings indicate that even though a population may have been significantly impacted by COVID-19, only a small portion of that population may actually have the presence of antibodies. This complicates the idea of herd immunity then given the fact that, in order to achieve it, usually around 60-70% of people in a population need to be immunized. Immunity can also be achieved through vaccination, however a COVID-19 vaccine is months away from completion. Achieving herd immunity without a vaccine would mean allowing millions of vulnerable people to get infected and potentially have serious health complications. 

It is still far too early for scientists to predict where COVID-19 falls on the immunity spectrum, but comparing it to other infectious diseases can lead us in the right direction. For now, what you can do to help scientists achieve this goal is to stay home and practice social distancing to keep yourself and your families safe.