Potential resurgence of respiratory syncytial virus in Canada

E1140 CMAJ | JULY 26, 2021 | VOLUME 193 | ISSUE 29 © 2021 CMA Joule Inc. or its licensors A ustralia and, more recently, the United States have seen a resurgence of cases of respiratory syncytial virus (RSV) infection in the summer season, as COVID-19 pandemic– related physical distancing measures are being relaxed. We argue that Canada should anticipate a similar resurgence in seasonal respiratory viruses in the summer of 2021. It is crucial that we continue to monitor respiratory illnesses to inform RSV prevention programs and help protect vulnerable patients. This may involve administering RSV monoclonal antibody therapy in high-risk children off season, which is a major departure from standard practice. Before the COVID-19 pandemic, lower respiratory tract infection caused an estimated 2.7 million deaths annually worldwide, ranking fourth in cause-specific mortality.1 Respiratory syncytial virus and influenza are leading causes of lower respiratory tract infection in young children. In children younger than 2 years, RSV presents as a bronchiolitis and, in younger infants, it presents with nonspecific symptoms like apnea and reduced oral intake. There is no RSV vaccine and no population-level intervention exists beyond basic hygiene measures. In Canada, high-risk children younger than 2 years — including infants born prematurely and those with severe chronic lung disease or serious congenital heart disease — currently receive monthly RSVneutralizing monoclonal antibody therapy during the RSV season (typically late fall to early spring in Canada),2 which reduces the risk of hospital admissions related to lower respiratory tract infection by more than half. This occurs annually on fixed dates based on past epidemiology, or on a variable season start based on active surveillance. Since the beginning of the pandemic, many countries have observed a near total disappearance of RSV and influenza cases.3,4 In Canada, 339 627 tests for RSV were reported through the Centre for Immunization and Respiratory Infectious Diseases. Of these tests, 239 were positive between Aug. 29, 2020, and May 8, 2021.5 In comparison, 412 861 tests for RSV were reported over a similar period in the previous year (Aug. 25, 2019, and May 2, 2020), of which 18 860 were positive. In response to the apparent disappearance of RSV during the pandemic, several Canadian RSV programs paused, shortened and even suspended the seasonal administration of monoclonal antibody, to avoid unnecessary medical visits.6 However, an interseason resurgence of RSV cases has recently been reported in the southern hemisphere in the setting of decreased SARS-CoV-2 circulation and relaxed physical distancing measures.7,8 The US Centers for Disease Control and Prevention also issued a health alert on June 10, 2021, about increased interseasonal RSV activity in the southern US since March 2021 (https://emergency.cdc.gov/han/2021/han00443.asp). The offseason resurgence in seasonal respiratory viruses now potentially poses a threat to vulnerable infants. The resurgence of cases of RSV in Australia and the US highlights the importance of population immunity in regulating RSV epidemics after exposure peaks. Studies have documented cyclical variations in maternally derived antibody levels, with an increase during the RSV winter season, followed by a decline over the summer months.9 During RSV seasons, most affected adults younger than 65 years remain asymptomatic or develop mild illness, owing to a lifelong seasonal exposure to the virus and memory Band T-cell immunity. In contrast, immunologically naive infants depend on passively transferred maternal antibodies to protect them at birth, but these antibodies wane within 6 months. Asymptomatic testing in households indicates that transmission may have continued to occur undetected during the COVID-19 pandemic,10 but it is more likely that during the pandemic, pregnant individuals were less likely to be exposed to RSV, and thus less likely to boost their RSV antibodies to levels usually seen in the winter. This raises a possibility that infants are less well protected than usual and could become sicker if they are infected this summer. COMMENTARY