herd of cows on a foggy morning standing on grass covid-19

4 COVID-19 Questions: What about herd immunity? Can cross-reactive immunity help?

Oct 28, 2020 COVID-19 Share:

Thinking about COVID-19? We are too.

Ever since the onset of the COVID-19 pandemic, every sector of our lives has been upended by changes brought about by this new virus. And in spite of extensive media coverage, it can be challenging to keep up with all of the questions brought about from living life in a global pandemic.

We want to bring some clarity to the confusion by discussing four questions you’re probably asking about COVID-19. To find out the answers to other questions about COVID-19, read the other posts in our series:

Could I already have some immunity to COVID-19?

We may be able to thank our school-aged kids for the annoying little cold germs they always seem to give their parents and teachers. While COVID-19 is a novel coronavirus, other coronaviruses circulate in our communities as a “common cold”. Data continues to accumulate about the presence of some immunity to COVID-19 from exposure to these other coronaviruses.

Immune responses to COVID-19

In the ongoing vaccine studies, researchers take the blood of someone that has contracted COVID-19 and recovered. Then the researchers expose the samples to the virus again to discover how their body fights the coronavirus. These same researchers expose the blood of people that have not had COVID-19 to the virus and assess the response of the immune system.

Researchers have discovered some fascinating data. In 40-60% of the samples that have not had COVID-19, researchers have seen an adequate immune response to the novel virus. This suggests that many people already have a form of immunity to COVID-19.1 This type of immunity is known as cross-reactive immunity.

Even more fascinating is that the mechanism used to kill the virus is different in the “immune” people who have not had COVID-19. These findings have been repeated in other vaccine studies, suggesting a starting level of immunity in a population that was previously unknown and not accounted for in public health interventions.

Understanding cross-reactive immunity

Cross-reactive immunity is immunity obtained to one virus because of a previous infection with a different virus, usually one from the same family. The immunity in individuals who have not been infected with COVID-19 comes in the form of CD4+ T cells. T cells, also called T lymphocytes, are an important part of the immune system. These T-cells of interest are commonly called ‘memory T cells’. Memory T cells are long-lived and can quickly expand to mount a proper immune response upon re-exposure to a remembered virus’s particles. By this mechanism, they provide the immune system with "memory" against previously encountered pathogens.

Older individuals do not produce ‘memory’ T-cells as readily as younger people. This might explain why older individuals are more likely to contract and have poor outcomes with COVID-19.

How cross-reactive immunity could help us reach herd immunity

In addition to finding a base level of cross-reactive immunity among individuals, the implications for herd immunity could be huge. You may have heard discussions about herd immunity for COVID-19. Herd immunity occurs when enough people within a population become immune to a disease to make further spread unlikely. An entire community can become protected from a disease, even if all of the individuals do not have personal immunity.

The threshold for developing herd immunity for illnesses like measles or mumps is typically 60-70% of the population infected. Many scientists are beginning to realize that this threshold may be markedly different in the context of a rapidly spreading pandemic where some people may carry some innate resistance. In the context of the pandemic, effective herd immunity could potentially be achieved with around 20% of the population.2 If this proves true, the likelihood of a second wave or resurgence of the virus will be very low.

Additionally, one under-appreciated outcome of rising case numbers is that, at this point, once a person contracts and recovers from COVID-19, they are no longer able to contract or transmit the virus. Therefore, they are no longer a possible spreader.

What now?

COVID-19 is a new virus. It’s normal to feel confused, especially when the messaging is often conflicting and biased. At MedHelp, we’re here for you when you have questions about your health. Established patients are always welcome to contact their physician with any questions they may have and receive an answer in about 24 hours.

Read the next post in our series about common COVID-19 questions, "What should we do about children and schools?"

Concerned about COVID-19?

We offer safe and convenient curbside testing for COVID-19 at each of our five locations. A telehealth screening is required before testing.


1Selective and cross-reactive SARS-CoV-2 T cell epitopes in unexposed humans Science 04 Aug 2020

2The impact of host resistance on cumulative mortality and the threshold of herd immunity for SARS-CoV-2: Jose Lourenco, Francesco Pinotti, Craig Thompson, Sunetra Gupta: medRxiv 2020.07.15.20154294; doi:10.1101


At MedHelp, it is our intent to provide valuable information and guidance for our patients. Covid-19 is rapidly evolving, and articles posted more than two months ago may not reflect the most up-to-date information about this illness. Please check our most recent articles for more current information.