Viral Transmission, Human Immunity & the Gap Called “Immunity Debt”

Viral transmission, human immunity & the gap called “immunity debt”

Paul Offit, MD: Hi, my name is Paul Offit. I'm talking to you today from the Vaccine Education Center at the Children's Hospital of Philadelphia. One thing that's happened recently is that we've seen an increase, a pretty dramatic increase in respiratory syncytial virus infections in our hospital. We've also seen a pretty large increase in influenza. And so, what some people wonder is, why? Why especially has there been an increase in respiratory syncytial virus infections?

And I think you can go back to 2020, because that was an unusual year. What happened in 2020 when SARS-CoV-2 virus came into our country, around January/February of 2020, we didn't have vaccines. We didn't have monoclonal antibodies. We didn't have antiviral agents. The only thing we had to try and protect ourselves from getting this virus was to limit our ability to interact with other people. And so, what did we do? We closed schools, we closed businesses, we restricted travel, we masked, we social distanced, we isolated, we quarantined, we tested and tested and tested, and we dramatically limited human-to-human contact. With that, we virtually eliminated respiratory syncytial virus, which is a common respiratory virus that we see every winter coming into our hospital, usually around October and November; we didn't see any respiratory syncytial virus infections. Similarly with influenza, usually influenza causes epidemics every year, where hundreds of thousands of people will be hospitalized and tens of thousands will die. That didn't happen in 2020. Normally, about 75 to 150 children die every year from influenza in this country. In 2021, one child died.

So, what that meant was we had successfully actually reduced the transmission of these respiratory viruses, like respiratory syncytial virus or like influenza virus, both of which can cause children to be hospitalized and suffer. And so, did that have an effect … did that have an effect? Did it create a so-called “immunity gap,” where when a virus circulates, it sorts of keeps boosting people's immunity so the following year they're somewhat better protected then against moderate or severe disease. And I think that is what happened. I think the reason that we, at the end of 2022, saw a dramatic increase in respiratory syncytial virus infections coming into our hospital and a dramatic increase in influenza was that you didn't have those viruses circulating two years ago. And similarly last year in 2021, also you … because we were still masking and social distancing and isolating and testing and trying to restrict our travel, you also didn't see as much of that virus, those viruses as we're seeing now.

So, I think there was this immunity debt. I think this will be the end of it. I think now with us going back basically to living and working and playing as we did before, that this will be the last year that that is likely to happen. But that is a consequence I think of what was such an unusual year in 2020.

Thank you.