The experience of RSV serves as a warning to be ready for future outbreaks, writes Cameron Grant.
The origins of this year’s RSV (respiratory syncytial virus) outbreak in children can be traced to the opening of the trans-Tasman bubble in April. By the end of May, five cases were recorded. Thereafter, each week that followed recorded an unprecedented jump. The cases each week were then 37, 110, 258, 590, 774, 892, with a peak of 952 cases in the third week of July.
The bubble opening brought international visitors to a population that due to the 2020 lockdowns and other measures to fight the Covid pandemic, had not experienced the usual winter season of respiratory viruses during 2020.
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Early childhood hospital admissions for respiratory viruses dropped significantly last year. For example, Kidz First Children’s Hospital had 159 Acute Lower Respiratory Infections (ALRI) hospital admissions among children younger than two years old, with RSV identified from only two respiratory samples. By comparison in each of the five years up to and including 2019, there were between 1486 and 2046 admissions a year and between 252 and 495 positive PCR tests for RSV.
New Zealand’s stringent interventions in response to the coronavirus mean the seasonal circulation of RSV and most other respiratory viruses in 2020 was interrupted. The numbers tell the story. Large reductions occurred in nation-wide testing for tested respiratory viruses. Compared with annual averages for 2015-2019, these reductions were influenza (99.9%), RSV (98.0%), metapneumovirus (92.2%), enterovirus (82.2%), adenovirus (81.4%), parainfluenza virus (80.1%) and rhinovirus (74.6%).
Multiple factors were at play. The international border controls, including mandatory 14-day isolation of arriving international travellers, limited the normal introduction of RSV into the country. Physical distancing and hygiene measures also contributed. Hand washing damages the lipid layer that surrounds RSV, reducing transmission.
In an ordinary year RSV is the leading cause of ALRI during early childhood. Globally in 2015, among children less than five years old, there were an estimated 33.1 million episodes of RSV-ALRI, with 3.2 million RSV-ALRI hospital admissions and 59,600 RSV-ALRI in-hospital deaths. In 2015, while more than 80 percent of RSV-associated ALRIs occurred in children older than six months, 45% of hospital admissions and in-hospital deaths were in children younger than six months of age.
Based on the New Zealand surveillance study, SHIVERS (Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance), at Starship Children’s and Kidz First Children’s Hospitals in Auckland from 2012-2015, the RSV-ALRI hospitalisation rate was 6.1/1,000 children from 0-4 years old during the 22-week surveillance period (May-September inclusive each year). Consistent with global data, 48 percent of the preschool age group RSV-ALRI hospitalisations were in children younger than six months.
When expressed as a rate per million population, the peak weekly RSV detection rates in 2021 were at least five-fold higher than those observed from 2015-2019. The age distribution also differed. As shown in data reported from the Laboratory Services at Middlemore Hospital, the number of RSV detections among children less than 12 months old were two-fold higher in 2021 versus 2019, whereas the number of RSV cases among children from 1 to 4 years of age were at least five-fold higher.
From a general respiratory virus perspective, what we have seen happen with RSV in 2021 may just be the first of several variances resulting from the impact of Covid-19 on the relationships between humans and respiratory pathogens.
In addition to the adult Covid-19 vaccination programme, the vaccine protection of children against other respiratory pathogens needs urgent attention. Similar to the increase in proportion of the population that is susceptible to more severe disease following exposure to RSV, we should anticipate and plan for influenza epidemics being larger and having a different age distribution.
There is the potential for children to be more vulnerable to future influenza epidemics. Influenza vaccine should be added to the schedule of vaccines for all children. New Zealand should also anticipate an increase of bacterial respiratory diseases, like pneumonia, should larger than expected epidemics of influenza occur.
Urgent attention is required to address the critical failure of our childhood immunisation programme if we are to prevent children experiencing epidemics of pertussis and measles to follow their, at times horrific, 2021 RSV experience.
The childhood immunisation schedule begins during pregnancy. Pregnancy doses of influenza and pertussis vaccine protect young infants against these diseases which frequently result in hospital admission when they occur in early infancy. The national coverage during pregnancy for women giving birth in 2018 was 31% for influenza vaccine and 44% for pertussis vaccine. From 2013-2018, the odds of Māori and Pacific women receiving either of these vaccines were lower than women of non-Māori and non-Pacific ethnic groups and were lower for women living in households in more deprived areas.
For the three months from April to June this year, only 75.3% of six-month old children in New Zealand were fully immunised. At age six months, coverage for the six-week, three-month, five-month scheduled vaccines among NZ European was 80.1%, among Māori, 54.9%, among Pacific, 68.7%, among Asian 93.0% and among children of other ethnic groups 78.8%. Coverage for children living in the most deprived quintile of households was 64% while for the least deprived quintile it was 83%.
New Zealand is entering an important phase of health reform. RSV provides us with insight into what to expect. From a health perspective we should plan for the unexpected. The worst-case scenarios are likely to be worse than what we have experienced. We must use the opportunity we now have to eliminate inequities in vaccine access based upon ethnicity, socioeconomic status, and age. Influenza, measles, pertussis, and invasive pneumococcal disease all have the potential to make our 2021 RSV experience appear relatively trivial in retrospect.
* This article is edited from an article by Professor Grant and co-authors published in the New Zealand Medical Journal, August 13, 2021.