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

Herd immunity occurs when a large portion of a community (the herd) becomes immune to a disease, making the spread of disease from person to person less likely. As a result, the whole community becomes protected, not just those who are immune. Often, a percentage of the population must be capable of getting a disease in order for it to spread. This is known as the herd immunity threshold. The percentage of a community that needs to be immune in order to achieve herd immunity varies from disease to disease. The more contagious a disease is, the greater the proportion of the population that needs to be immune to the disease to stop its spread. For example, the measles is a highly contagious illness. It is estimated that 94% of the population must be immune to interrupt the chain of transmission.

There are two paths to herd immunity for COVID-19: vaccines and infection. Vaccination for the virus that causes COVID-19 is the best approach to achieving herd immunity. Vaccines create immunity without causing illness or resulting complications. Herd immunity makes it possible to protect the population from a disease, including those who can't be vaccinated, such as newborns or those who have compromised immune systems. Using the concept of herd immunity, vaccines have successfully controlled deadly contagious diseases such as smallpox, polio, diphtheria, rubella and many others.

Reaching herd immunity through vaccination sometimes has drawbacks. Protection from some vaccines can wane over time, requiring revaccination. Sometimes people do not get all of the shots that they need to be completely protected from a disease. In addition, some people may object to vaccines because of religious objections, fears about the possible risks or skepticism about the benefits. Others mistakenly believe that because these vaccines are produced by a new platform they want to wait until their friends and family members have been vaccinated to determine the efficacy and safety of the vaccines before receiving them. People who object to vaccines often live in the same neighborhoods or attend the same religious services or schools. Also, young people in the age range of 20 to 50 have the assumption that this virus produces less severe disease in their age group so that they do not need to be vaccinated. However, newly evolving emergent variant viruses have been shown to produce more sever disease in individuals in this younger age group. Opposition to vaccines has spread through social media using erroneous or unproved data. Vaccine opposition or hesitancy can pose a real challenge to herd immunity. If the proportion of vaccinated people in a community falls below the herd immunity threshold, exposure to a contagious disease could result in the disease quickly spreading. Measles has recently resurged in several parts of the world with relatively low vaccination rates, including the United States.

Herd immunity can also be reached when a sufficient number of people in the population have recovered from a disease and have developed antibodies against future infection. However, there are some major problems with relying on community infection to create herd immunity to the virus that causes COVID-19. First, it is not yet clear if infection with the COVID-19 virus makes a person immune to future reinfection. Research suggests that after infection with some coronaviruses, reinfection with the same virus, though usually mild and only happening in a fraction of people, is possible after a period of months. Further research is needed to determine the longevity for the protective effect of antibodies to the virus in those who have been infected.

Even if infection with the COVID-19 virus creates long-lasting immunity, a large number of people would have to become infected to reach the herd immunity threshold, which would result is an increase in already unacceptable mortality levels. Experts estimate that in the U.S., 70% to 80% of the population, more than 220 million people, would have to recover from COVID-19 or be vaccinated to halt the epidemic. Presently only 25% of the U.S. population have received at least one dose of vaccine, and an accurate number of people that have been infected is not known, because a significant number of infection cases have never been reported to local health agencies.

It is crucial to slow the spread of newly emerging variant COVID-19 viruses by all those ages 16 or older receiving a vaccine as soon as it is possible in their area and continue to practice preventative mitigations. Vaccines have been shown to produce a higher, longer-lasting immunity to the virus than natural infection. Individuals should keep social distance between themselves and others (within about 6 feet), especially those who have respiratory symptoms or have not been vaccinated. This is especially important if you have a higher risk of serious illness. Some people may have the COVID-19 virus and spread it to others, even if they do not have symptoms or do not know they have COVID-19. Avoid large crowds in indoor places that have poor ventilation. Wash your hands often with soap and water for at least 20 seconds or use an alcohol-based hand sanitizer that contains at least 60% alcohol when touching objects in public places. Cover your face with a cloth or surgical mask, or even double mask, in public spaces, such as the large grocery stores and sporting or concert events, where it is difficult to avoid close contact with others. Stay home from work, school and public areas for at least 10 days if you have been exposed to a person that has tested positive for COVID and 14 days if you have a respiratory disease and tested positive for COVID and notify all those that you have been in contact with.

These preventative mitigations should be enacted even after a person has been vaccinated, because vaccines are not 100% effective and vaccine efficacy may be reduced against continually emerging new variants. Remember current vaccines are free and safe and soon to be available to all age groups over 16, and prompt vaccination is necessary to reduce the emergence of new highly transmissible variants, which can cause more serve disease in all age groups. We are now at a turning point with this pandemic where there is a race between vaccination and the rapid spread of viral variants. Recent relaxation of mitigations, as is currently being enacted in many states, will no doubt lead to a fourth wave of infections, which could last through the entire summer. Results from the increase in the scope of the pandemic during the summer months in 2020 indicated that this coronavirus, unlike influenza viruses, is not seasonal nor adversely affected by increases in temperature or humidity. In fact, the worse uptake in viral infections occurred immediately following summer holidays.

About Joseph Giambrone:

Joseph Giambrone is a professor emeritus in Auburn University’s Department of Poultry Science with a joint appointment in the Department of Pathobiology in the College of Veterinary Medicine. During his graduate research career at the University of Delaware, he was part of a research group that developed the first vaccine against an antigenic variant of an avian coronavirus. During a sabbatical leave during his tenure at Auburn, he was part of a research group in Australia that sequenced the entire genome of antigenic variant of a coronavirus of chickens. During his 42-year research career as a molecular virologist, immunologist and epidemiologist, he has made critical advancements in understanding the ecology of viral pathogens, led efforts to improve detection and surveillance of viral diseases and developed new and effective vaccines and vaccine strategies to protect commercially reared chickens as well as pathogens, such as avian influenza viruses, which have spilled over into human populations. His research has had a profound impact on practices used today to reduce the incidence and severity of viral diseases of commercially reared poultry as well in human populations.