SciMoms COVID FAQs: Immunity and Herd Immunity

In this series of COVID FAQs, the SciMoms answer your frequently asked questions about the novel coronavirus or COVID-19 pandemic. In this section, Alison and Layla focus on COVID-19 immunity and herd immunity.

What is an antibody? 

Antibodies are proteins produced by the immune system when specialized cells detect the presence of something that they do not recognize as part of your body. An antigen is a foreign substance that triggers antibody production. These include, but are not limited to, viruses, bacteria, or proteins that trigger allergic reactions. Antibodies recognize specific antigens. 


What are antibodies?

National Human Genome Research Institute Glossary: Antibody

Why are antibodies important for immunity?

Antibodies are the critical mediators of adaptive (or acquired) immunity.

The immune system is divided into two categories — innate and adaptive. The innate immune system includes defenses that are quickly activated when an antigen enters the body. This portion of the immune system responds generally to infection and does not react to specific antigens or pathogens.

In contrast, adaptive (or acquired) immunity is the body’s response to antigens that it has seen before. Antibodies are the critical mediators of the adaptive immune system. The first time your immune system sees an antigen, it takes time to produce specific antibodies that bind to and recognize this new antigen. If this antigen is seen again, the cells can produce antibodies more quickly and attack the antigen more efficiently. This “memory” of the adaptive immune system prevents you from getting certain diseases more than once and makes subsequent infections less severe.

We can gain adaptive immunity by infection with the actual disease (natural immunity) or vaccination (vaccine-induced immunity). Vaccination allows our bodies to develop adaptive immunity without getting sick. Vaccines include inactive antigens from disease-causing pathogens. These inactive antigens can be dead or weakened pathogens or proteins that are not infective. These antigens train the immune system to produce specific antibodies against the virus or bacteria. If you are infected after vaccination, your body already has acquired this “memory” and can produce antibodies to fight off the infection and prevent illness.


Immune System (by the Amoeba Sisters)

COVID-19 Immunology 101 for Non-immunologists by Akiko Iwasaki, Ph.D.

What are COVID antibodies?

COVID antibodies are antibodies produced against SARS-CoV2, the virus that causes COVID-19. Antibody tests look for the presence of COVID antibodies in a blood sample. These tests tell you if you have had a past infection of SARS-CoV2, but testing for antibodies is not a diagnostic test for COVID-19 (see our FAQ on diagnostic tests). For more information on antibody tests, please visit these resources.


Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers

Periodic Graphics: A guide to COVID-19 testing

Antibody tests, part 1: What can antibody tests tell us?

Antibody tests, part 2: How do antibody tests work?

What do we know about COVID-19 immunity?

Scientists are still trying to understand immunity to SARS-CoV2. Because this is a new virus, we aren’t exactly sure of the parameters of immunity, such as how long it takes to develop an immune response and the length of time a person is immune after having the virus. Unfortunately, we can only learn these things over time as scientists and doctors observe and study patients who have recovered from COVID.

However, SARS-CoV2 is a coronavirus, and we do know some things about immunity to coronaviruses. Scientists can use that information to make some baseline guesses. With other human coronaviruses, people develop immunity for up to 2 years after infection, with immunity declining over time. Based on this, scientists expect that we will have an immune response and develop temporary immunity.  At this point, it seems that people do mount immune responses and, based on other coronaviruses, probably have some short-term immunity, but we do not know yet how long that immunity might last. Encouraging new data suggests that the immune system is acting in ways that are consistent with long-lasting immunity.

According to the WHO, most people develop an immune response within a few weeks of infection. Even people who were asymptomatic seem to develop an immune response. There is some data showing that people still produce antibodies seven to nine months after infection. It is not possible to know how long these antibodies will last beyond this time frame since the earliest cases of SARS-CoV2 infection were not even a year ago. Scientists continue to monitor how long people produce antibodies. 

Areas of ongoing COVID-19 immunity research include the following: 

  • Does the presence of antibodies against SARS-CoV2 provide immunity? How long does this protection last?
  • Does the severity of disease (asymptomatic, mild, severe) relate to strength or duration of immunity?
  • Do genetic or biological variables (like age) affect immunity?
  • Which types of antibodies are important for COVID-19 immunity?  

Understanding how our immune systems respond to the SARS-CoV2 virus is essential for vaccine development and planning public health measures to control the spread.


How long does immunity last after COVID-19? What we know

WHO FAQ: What do we know about immunity from COVID-19?

Can people be reinfected with SARS-CoV2?

Alarming news stories abound about people getting COVID-19 for a second time. However, the data suggest this is unlikely and quite rare. SARS-CoV2 appears to behave like other viruses in this regard. While it might be possible for the virus to reinfect someone, this is unlikely to occur shortly after the first infection and also unlikely to make them sicker the second time because of the adaptive immune system. However, not everyone’s immune response is the same so it is possible that some people may be able to be reinfected earlier. The take-home message here is to keep practicing the appropriate prevention measures, even if you have previously been infected.


Can I get RE-infected with COVID-19? — Dear Pandemic

Dear Pandemic – 10/17/20 NG Live Q&A (Reinfection (2:25 – 7:45))

Can You Get Covid-19 Again? It’s Very Unlikely, Experts Say

Can I boost my immune system to protect myself against COVID?

There is a vast market for products that “boost your immune system.” Many companies selling these products have jumped at the opportunity to sell more products by claiming that they can protect you against COVID. But what does it actually mean to “boost your immune system”? 

Is it boosting your innate immune system? A boosted innate immune system would leave you continually feeling unwell because this system includes a range of nonspecific defenses that produce symptoms like fever, flushing, sore throat, joint pain, or runny nose. In addition to feeling sick all the time, this would also not mean that you are less susceptible to a subsequent infection.

Is it boosting your adaptive immune system? This is what vaccines do by helping your immune system produce antibodies against a pathogen before you get infected. If someone is selling a product that boosts your adaptive immune system, they would be selling an untested and unregulated vaccine. 

An overactive immune system can also cause allergies and auto-immune diseases. In these disorders, our immune system fights against things that it shouldn’t, like pollen or food (allergies) or our own tissues (auto-immune diseases).

Even though no specific diet or supplement is known to make your immune system work better, we do know that following general health recommendations go a long way to reducing disease risk.


What can you do to improve your immune system? – Harvard Health

Why bingeing on health foods won’t boost your immune system

Boost your immunity: Cold and flu treatments suppress innate immune system

What is herd immunity?

Herd immunity means that a sufficient number of people in the population are immune and no longer susceptible to a disease so that those who are susceptible are indirectly protected.  

For a virus to spread, it needs susceptible people to infect. If people are immune, they are no longer susceptible and cannot be infected. When enough people in a population are immune, the virus cannot spread, and a new outbreak cannot start — this is herd immunity. The percentage of people who must be immune to reach this level is called the herd immunity threshold.

What is herd immunity? (British Society for Immunology)

Typically, when 70-90% of the population is immune, herd immunity is achieved. The herd immunity threshold depends on the characteristics of the virus. Highly contagious viruses, such as measles, need a very high level of herd immunity (>95%) to stop the virus from spreading in a community. Herd immunity is reached at a lower percentage for less contagious pathogens (i.e., have a lower R-nought, which is a measure of a pathogen’s potential to spread).  

Relationship between disease contagion and herd immunity threshold (National Foundation for Infectious Diseases)

For SARS-CoV2, scientists have struggled to estimate the herd immunity threshold because there is still much unknown about the virus. However, scientists can use the data we have to make some estimates. Most estimates of the herd immunity threshold for this virus are between 60-70% of the population, with lower estimates around 40%.


“Herd Immunity”: A Rough Guide | Clinical Infectious Diseases

The Simple Math of Herd Immunity

How do we achieve herd immunity?

There are two paths to herd immunity — natural infection and vaccines. 

Herd immunity through natural infection involves allowing a disease to spread through a population until enough people recover and have immunity to protect susceptible individuals.

Herd immunity through vaccination provides immunity without the need for illness and disease-related complications. To achieve herd immunity through vaccination, enough people in a population must receive the vaccine to protect those who are too young or are unable to be vaccinated, and people in whom the vaccine has failed.

Because immunity can decline over time and the population continues to grow, measures must be taken to maintain herd immunity over time. For example, booster shots are needed for certain vaccines, and vaccination programs are necessary as the population grows. With naturally acquired herd immunity, maintaining herd immunity would require repeated rounds of infection throughout the community.


Herd immunity and COVID-19 (coronavirus): What you need to know

Have we ever controlled a disease through herd immunity?

Before we answer this question, let’s clarify definitions for controlling infectious diseases. 

Eradication means that worldwide transmission of a disease is over with no remaining cases. When a disease is eradicated, further intervention measures are unnecessary. We have only entirely eradicated one human disease — smallpox — due to a global immunization program led by the WHO. The last known natural case of smallpox was in 1977.  

Elimination means that transmission within a geographic area is no longer active. When a disease is eliminated, intervention measures like vaccination are still necessary to keep the disease under control. Interventions are still required because the disease is still present in other areas, and people travel between those places. Elimination is also time-constrained: if an outbreak continues for a year, the disease is no longer considered to be eliminated. 

Measles was declared eliminated from the US in 2000, but we have had multiple outbreaks of measles since then in pockets of unvaccinated people from travelers to the US from places that still have measles. Until a disease is eradicated — meaning it is eliminated everywhere — vaccines and other preventive measures are necessary to prevent outbreaks.

Now that we have those definitions explained, we can answer the question. 

We have never achieved herd immunity and eliminated or eradicated a disease through natural infection. We can achieve herd immunity and control diseases through vaccination but have only ever eradicated one disease in people — smallpox. Other diseases have been eliminated in some countries, but we still find reservoirs of disease in areas with lower vaccination rates.

When public health officials express concern about losing herd immunity for vaccine-preventable illnesses, it means that too many people are not immune, so that outbreaks are possible, and the disease can spread. When the percentage of people who are protected falls, the virus can spread to susceptible hosts and start a new outbreak, as we have seen in recent years with outbreaks of measles and pertussis


Eradication of Diseases

Should I get other vaccines for my children and me during the pandemic?

Amid the COVID-19 pandemic, we find ourselves at risk of declining herd immunity against many vaccine-preventable illnesses. The American Academy of Pediatrics recommends that people stay up to date on vaccinations for themselves and their kids. The President of the AAP issued the following statement in May 2020:

“We know parents are worried. We want to reassure all our families that pediatricians have innovated ways to make visits even safer, including setting different hours or locations for well and sick children, rigorous sanitation and cleaning practices, and conducting portions of visits by telehealth.  The American Academy of Pediatrics urges all parents to contact their pediatrician to schedule a visit to catch up on vaccines or for a well-child check-up. AAP has published new recommendations today to guide pediatricians in managing visits safely and effectively.”

This year, more than ever, it is critical to get the flu vaccine. Parents often forgo this shot, but due to COVID, there is a risk that we might end up with two epidemics if we do not protect our communities against the flu (this has been called a “twindemic” by some outlets). With our medical resources overburdened by COVID, our medical system will be unable to support a surge of flu patients. Protect your family and your community by getting a flu shot as soon as possible. 


AAP urges vaccination as rates drop due to COVID-19

Protect Your Health This Season

Is natural herd immunity a good strategy to end the COVID-19 pandemic?

To achieve herd immunity without a vaccine, enough people need to contract COVID-19 to thwart the virus’s ability to spread. This necessarily translates to a substantial portion of our population contracting COVID-19, overwhelming our hospitals, and putting people purposefully and unnecessarily in harm’s way. Adopting a natural herd immunity strategy is unethical, based on bad science, and fails to hold the government accountable for their failure to take the necessary public health and economic measures to protect our health and economic well-being.

Health officials like the Director-General of the World Health Organization have warned that attempting to achieve herd immunity would be unethical due to the unnecessary illness, long-term complications, and mortality that would result. Even attempting to achieve natural herd immunity would mean knowingly and intentionally killing swaths of people, and it would be plain wrong.

Although those who support this approach correctly highlight that high-risk groups should be protected, they erroneously assume that asymptomatic individuals or people with mild symptoms are not impacted. Those who support a natural herd immunity strategy rely solely on the relatively low death rate among healthy individuals to justify their lack of precautions.  However, the long-term impact of COVID-19 infection in survivors, even if they were not in a high-risk group, is abundantly clear. Many asymptomatic individuals are suffering from heart damage, and many young people have long-lasting health impacts. Groups that have been disproportionately impacted by pollution or poor work conditions are more often harmed by this virus. Due to systemic racism and health disparities, this virus ends up disproportionately harming Latino and Black families

Herd immunity by vaccination is a vastly different scenario. This is needed and desirable but should not be confused with achieving herd immunity through natural infection. Letting the virus infect our population translates to hundreds of thousands more dead, millions with long-lasting and severe health complications, and worsening health disparities. Vaccination is the safest way to achieve herd immunity.

Those who support a natural approach to herd immunity create a false dichotomy between allowing the virus to spread unchecked and total lockdown. There are many options in between that would control the spread of the disease. No one is actually proposing that we control the pandemic through endless lockdowns. Instead, scientists have recommended first managing community transmission and then preventing new outbreaks through comprehensive testing, contact tracing, and interventions like implementing mask mandates, improving indoor ventilation, and encouraging strict adherence to the 3Ws (wear a mask, watch your distance, and wash your hands). In most areas of the US, we never got community spread under control and never implemented the appropriate additional measures to allow us to safely emerge from lockdown. Only when community spread is contained is it possible to emerge from lockdown and use these measures to control new outbreaks. 


The false promise of herd immunity for COVID-19

Scientific consensus on the COVID-19 pandemic: we need to act now

Covid-19 herd immunity, explained

The lasting misery of coronavirus long-haulers

The herd immunity strategy for Covid-19 is a nightmare

The ‘herd immunity’ route to fighting coronavirus is unethical and potentially dangerous

The Concept of Coronavirus Herd Immunity Is Deadly and Dangerous