The amount of coughing in the rink lobby suggests the puck dropped weeks ago on cold and flu season. Should parents be worried about carrying on with indoor sports like hockey? Learning to live with respiratory infections again requires an understanding of how immunity is developed, and a fast-moving strategic sport like hockey provides a good visual.
Most children now have some immunity to the virus which causes Covid-19 (SARS-CoV-2) and a wide variety of other seasonal viruses. To understand why so many people are sick right now, we need to first separate the player from the puck: infection and disease are different.
Infection vs disease
Immunity against infection is earned the hard way—by developing airway-lining antibodies after recovering from illness. This is why “natural” immunity to Covid is so effective (here, here) but also risky for the elderly or those with a serious medical condition.
Vaccination, on the other hand, may not prevent infection by SARS-CoV-2 or the flu virus but it can reduce the chances of progressing to severe disease. Having antibodies in the blood helps guard the “slot”—the heart and lungs—after infection. Here’s how the immune system responds to a viral “breakout.”
Neutral zone defense during infection
To visualize the layers of defense against infection, follow the puck (SARS-CoV-2) from left to right as immune defenders swing into action. The same process occurs after exposure to other viruses sweeping the country now.
The battle starts in the mucus membranes lining the nose, mouth and throat (blue area below). During the first few days after infection, the innate immune system swings into action with generalist defenders. These agents need no special training because their job is to recognize “self” vs “non-self” antigens. Fever and fatigue are signs that the immune system is ramping up in response to an infection.
The immune system has special teams, too
Within about two weeks, trained defenders are ready to take a shift. These specific antibodies bind to the virus before it gets to the vital organs deep in the defensive zone (yellow). The first (and largest) antibody to form is IgM, followed by IgG and IgA. In addition to generating antibodies against the spike protein like vaccination does, people who have recovered from Covid-19 also have antibodies to the nucleocapsid (N) protein.
When the battle is over, the debris is picked up by macrophages and deposited in the lymphatic system (such as the tonsils and lymph nodes under the arm), which is eventually drained to the gut with hydration and exercise.
The center sets up critical plays
The center wins faceoffs, sets up plays and drives breakouts. After recovering from a cold, specialized IgA antibodies (SIgA) lining the mucus membranes play this role, too. This extra layer of protection in your upper airways is called “mucosal immunity” and is built after recovering from a cold. This is also why a nasal vaccine may be very helpful for Covid (FluMist is a nasal flu vaccine).
Defensive pairs are highly coordinated
The D lines have good chemistry, communicate well, and are able to skate backwards quickly to protect the slot. The memory B cells and T cells (circles above) play this role. The memory B cells produce antibodies against future attacks. The T cells stay on patrol and give the B cells a heads-up about which antibodies to make. They also tell infected and precancerous cells to self-destruct. Although antibodies wane over time, the T cells stay relatively stable and recognize variants.
Just as a coach tries different lines, so too are antibodies adjusting, trying to anticipate the next change in the virus. After Covid infection, special areas of our immune system called germinal centers continue to train the antibody response by creating mutations and testing them to find the best “fit.” This intense training by antibody-forming cells takes months to refine—and is why a longer interval (i.e., more training time) between vaccine doses induces a higher quality antibody response.
Why “breakthrough” happens after vaccination
The mucus membranes of the nose and upper airway membranes—the body’s neutral zone—come in contact with everything we eat and breathe. Infection builds mucosal immunity, but vaccination does not trigger enough of the airway-lining antibodies to prevent infection. Infection is the final phase of immune development rather than vaccine failure and induces broad protection against many different variants.
The long game
Building immunity through infection carries the risk of serious disease which is highly variable depending on age and health status. Particularly for higher risk infants, children and adults, setting up defensive zone coverage by getting vaccinated or boosted before encountering either Covid or flu virus is like putting a helmet and pads on before hitting the ice. Having a high level of antibodies helps to protect vital organs like the lungs, and boosting elicits a blitz of antibodies to neutralize the virus.
The original mRNA vaccines elicit good humoral (antibody) and cellular (T and B cell) protection which is important for preventing severe disease. The flu vaccine, in contrast, must be adapted every year to match the circulating strain of influenza because it depends on antibodies for protection and does not do a good job stimulating long-lasting cellular (T and B cell) immunity.
Should kids be boosted?
A Covid variant-specific bivalent (2-strain) booster may produce more neutralizing antibodies against Omicron than the original vaccine did (here and here), but the clinical importance is unknown, especially among young, healthy people who have recovered from SARS-CoV-2 infection. In fact, recent studies (here, here, and here) suggest that boosting may diminish protection against infection by over-training on an outdated antigen.
In addition, myopericarditis is a rare side effect of mRNA vaccination which affects young males in particular. For this reason, it is important to weigh risks and benefits of Covid vaccination with an individual’s unique health history in mind and with a trusted healthcare professional.
Smart defense at this stage of the pandemic means vaccinating high risk people who are not yet immune, and vaccine experts like Paul Offit, MD, suggest that we can be strategic about who benefits most from a bivalent Omicron booster: the elderly over age 65, and children and adults with serious long-standing health conditions, and the immune compromised.
A winning game plan provides the team with the best strategy, line chemistry, and endurance. After recovering from infection, our immune system airway-lining antibodies and trained defenders on patrol. This is why natural immunity to SARS-CoV-2 is so durable, and why vaccinating high-risk people for Covid or flu before infection is heads-up hockey.
Allison Krug MPH is an epidemiologist, policy analyst and COVID Liaison for a non-profit youth ice hockey club in Virginia Beach. Lucy McBride MD is a practicing internist and author of a popular COVID newsletter and a Washington Capitals fan. Daniel Johnson MD is Chief of Pediatric Infectious Diseases at the University of Chicago School of Medicine and played hockey as a kid. The authors are grateful for expert review and helpful discussion with Alessandro Sette Dr.Biol.Sci., who has studied immunology for 35 years and leads the Sette Lab at the La Jolla Institute for Immunology.