April 29 is the International Day of Immunology, a day that “is dedicated to increasing global awareness of the importance of immunology in the fight against infection, autoimmunity and cancer,” according to the International Union of Immunological Societies.
This year, with the COVID-19 crisis still raging in most of the world, understanding immunity – what it means, how and when it works – is more relevant than ever, said Prof. Dror Mevorach, chairman of medicine and director of the Rheumatology Research Center at Hadassah-University Medical Center.
In five questions and answers, here is what you need to know about immunity.
1. What is immunity?
Immunity is the body’s response to different triggers.
“The immune system mediates many processes in the body that previously were thought not to be related to the immune system, and definitely [including] processes like infection that usually trigger an immune response,” Mevorach explained.
But he said that with COVID-19 there is a problem: the immune response is more dangerous than the virus itself.
“It is like an army that can do a lot of harm in the name of fighting against the intruder,” Mevorach said. “Sometimes we don’t need these cannons and missiles to fight an intruder that will pass away soon. And in the case of COVID-19, some people’s immune response is exaggerated and, as a result, they get some ultra-inflammation called a cytokine storm that can actually cause deterioration of the medical situation.”
2. There are two types of immunity: innate and adaptive. What is the difference?
Innate immunity is a response that is not specific to molecular structure, Mevorach said. In contrast, adaptive immunity can identify a foreign agent and develop specific T cells that will eliminate this antigen, or specific B cells that will produce specific antibodies against it.
He said the innate immune system is nonspecific, meaning that it targets anything identified as foreign. Adaptive immunity is specific to the particular pathogen that induced it. A person who recovers from COVID-19 is protected against reinfection by the adaptive immune system. But that immunity would likely not protect him against other coronaviruses, and it would definitely not work against other unrelated viruses, like measles or mumps.
3. When we talk about vaccine immunity, what do we mean?
“We are talking about imitating a part of the virus. Sometimes it is a real virus, sometimes it is a deactivated virus, and sometimes – as in the new technology of mRNA vaccination – it is a sequence of RNA codes that relate to a part of the virus,” Mevorach said. “With Pfizer and Moderna, the mRNA vaccines are coded for the spike protein.
“When the vaccine enters your body, it elicits an immune response and develops T cells and B cells against the spike protein,” he continued. “Then, when you encounter the virus a few weeks or months later, you have a kind of immunization army waiting to eradicate the disease.”
4. How do we know if we personally developed immunity?
There are two ways: one is the theoretical way, the other is the practical way.
“The theoretical way is that when examined, it was shown that 95% of people immunized by the Pfizer or Moderna vaccine became immunized. So, statistically, you can assume that if we immunize 100 people, 95 of them will be immunized against coronavirus,” Mevorach said.
In contrast, if one wants to practically determine if he is immune to a certain virus, then the person’s antibodies need to be measured, such as through a serological test, and it needs to be verified that the person has in fact developed B cells and T cells.
5. We hear a lot about herd immunity. What is it and how is it achieved?
Mevorach explained that herd immunity is the situation where enough individuals are immune to a disease, either through getting sick or vaccination. Usually, it takes too many sick people to develop natural herd immunity, and this can be dangerous, since some people will get extremely sick and even die in the process. With vaccination, when enough people are vaccinated, they become immune and then the disease cannot spread.
“Let us say someone is sick and encounters nine people, but all are immunized,” Mevorach said. “The disease will not spread.”
A year ago in Israel, when a person who was infected with COVID-19 encountered 10 people, there was a risk that all 10 would contract the virus because it spread so quickly. Now, with more than five million people inoculated, that same sick person can encounter 10 people, but most likely they already developed immunity and therefore the disease does not spread.
“When you get to a place where the disease does not spread at all anymore, that is herd immunity,” according to Mevorach. “We are aiming for worldwide herd immunity, but that will take a long time. In Israel, we are on our way to getting it – and hopefully it will occur in the United States, the United Kingdom and some other vaccinated countries soon, too.”
Mevorach added that even when a society develops herd immunity, there still can be a few outbreaks among unvaccinated segments of the population.