ID Consult

Immunity debt and the tripledemic


Respiratory syncytial virus (RSV) and influenza cases are surging to record numbers this winter in the wake of the COVID-19 pandemic when children were sheltering in the home, receiving virtual education, masking, and hand sanitizing, and when other precautionary health measures were in place.

Dr. Michael E. Pichichero, a specialist in pediatric infectious diseases, and director of the Research Institute at Rochester (N.Y.) General Hospital

Dr. Michael E. Pichichero

RSV and flu illness in children now have hospital emergency rooms and pediatric ICUs and wards over capacity. As these respiratory infections increase and variants of SARS-CoV-2 come to dominate, we may expect the full impact of a tripledemic (RSV + flu + SARS-CoV-2).

It has been estimated that RSV causes 33 million lower respiratory infections and 3.6 million hospitalizations annually worldwide in children younger than 5 years old (Lancet. 2022 May 19. doi: 10.1016/S0140-6736(22)00478-0). RSV is typically a seasonal respiratory infection occurring in late fall through early winter, when it gives way to dominance by flu. Thus, we have experienced an out-of-season surge in RSV since it began in early fall 2022, and it persists. A likely explanation for the early and persisting surge in RSV is immunity debt (Infect Dis Now. 2021 Aug. doi: 10.1016/j.idnow.2021.05.004).

Immunity debt is an unintended consequence of prevention of infections that occurred because of public health measures to prevent spread of SARS-CoV-2 infections. The COVID-19 lockdown undoubtedly saved many lives. However, while we were sheltering from SARS-CoV-2 infections, we also were avoiding other infections, especially other respiratory infections such as RSV and flu.

Our group studied this in community-based pediatric practices in Rochester, N.Y. Physician-diagnosed, medically attended infectious disease illness visits were assessed in two child cohorts, age 6-36 months from March 15 to Dec. 31, 2020 (the pandemic period), compared with the same months in 2019 (prepandemic). One hundred forty-four children were included in the pandemic cohort and 215 in the prepandemic cohort. Visits for bronchiolitis were 7.4-fold lower (P = .04), acute otitis media 3.7-fold lower (P < .0001), viral upper respiratory infections (URI) 3.8-fold lower (P < .0001), and croup 27.5-fold lower (P < .0001) in the pandemic than the prepandemic cohort (Front Pediatr. 2021 Sep 13. doi: 10.3389/fped.2021.72248).

The significant reduction in respiratory illness during the COVID-19 epidemic we and others observed resulted in a large pool of children who did not experience RSV or flu infections for an entire year or more. Herd immunity dropped. The susceptible child population increased, including children older than typically seen. We had an immunity debt that had to be repaid, and the repayment is occurring now.

As a consequence of the surge in RSV, interest in prevention has gained more attention. In 1966, tragically, two infant deaths and hospitalization of 80% of the participating infants occurred during a clinical trial of an experimental candidate RSV vaccine, which contained an inactivated version of the entire virus. The severe side effect was later found to be caused by both an antibody and a T-cell problem. The antibody produced in response to the inactivated whole virus didn’t have very good functional activity at blocking or neutralizing the virus. That led to deposition of immune complexes and activation of complement that damaged the airways. The vaccine also triggered a T-cell response with inflammatory cytokine release that added to airway obstruction and lack of clearance of the virus. RSV vaccine development was halted and the bar for further studies was raised very high to ensure safety of any future RSV vaccines. Now, 55 years later, two RSV vaccines and a new preventive monoclonal antibody are nearing licensure.

GlaxoSmithKline (GSK) and Pfizer are in phase 3 clinical trials of a safer RSV vaccine that contains only the RSV surface protein known as protein F. Protein F changes its structure when the virus infects and fuses with human respiratory epithelial cells. The GSK and Pfizer vaccines use a molecular strategy developed at the National Institutes of Health to lock protein F into its original, prefusion configuration. A similar strategy was used by Pfizer/BioNTech and Moderna in their design of mRNA vaccines to the SARS-CoV-2 spike surface protein.

A vaccine with the F protein in its prefusion form takes care of the antibody problem that caused the severe side-effects from the 1966 version of inactivated whole virus vaccine because it induces very high-efficiency, high-potency antibodies that neutralize the RSV. The T-cell response is not as well understood and that is why studies are being done in adults first and then moving to young infants.

The new RSV vaccines are being developed for use in adults over age 60, adults with comorbidities, maternal immunization, and infants. Encouraging results were recently reported by GSK and Pfizer from adult trials. In an interim analysis, Pfizer also recently reported that maternal immunization in the late second or third trimester with their vaccine had an efficacy of 82% within a newborn’s first 90 days of life against severe lower respiratory tract illness. At age 6 months, the efficacy was sustained at 69%. So far, both the GSK and Pfizer RSV vaccines have shown a favorable safety profile.

Another strategy in the RSV prevention field has been a monoclonal antibody. Palivizumab (Synagis, AstraZeneca) is used to prevent severe RSV infections in prematurely born and other infants who are at higher risk of mortality and severe morbidity. Soon there will likely be another monoclonal antibody, called nirsevimab (Beyfortus, AstraZeneca and Sanofi). It is approved in Europe but not yet approved in the United States as I prepare this column. Nirsevimab may be even better than palivizumab – based on phase 3 trial data – and a single injection lasts through an entire normal RSV season while palivizumab requires monthly injections.

Similar to the situation with RSV, the flu season started earlier than usual in fall 2022 and has been picking up steam, likely also because of immunity debt. The WHO estimates that annual epidemics of influenza cause 1 billion infections, 3 million to 5 million severe cases, and 300,000-500,000 deaths. Seasonal flu vaccines provide modest protection. Current flu vaccine formulations consist of the hemagglutinin (H) and neuraminidase (N) proteins but those proteins change sufficiently (called antigenic drift) such that production of the vaccines based on a best guess each year often is not correct for the influenza A or influenza B strains that circulate in a given year (antigenic mismatch).

Public health authorities have long worried about a major change in the composition of the H and N proteins of the influenza virus (called antigenic shift). Preparedness and response to the COVID-19 pandemic was based on preparedness and response to an anticipated influenza pandemic similar to the 1918 flu pandemic. For flu, new “universal” vaccines are in development. Among the candidate vaccines are mRNA vaccines, building on the success of the SARS-CoV-2 mRNA vaccines (Science. 2022 Nov 24. doi: 10.1126/science.abm0271).

Dr. Pichichero is a specialist in pediatric infectious diseases, Center for Infectious Diseases and Immunology, and director of the Research Institute, at Rochester (N.Y.) General Hospital. He has no conflicts of interest to declare.

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