The Americas Vastly Under-Counted Zika Cases

While several hundred thousand Zika cases have been reported across the Americas since 2015, the actual incidence of infection was likely much higher.

A new study published on September 28, 2020, found due to a high frequency of asymptomatic infection and other challenges that surveillance systems faced, most Zika virus cases were never counted.

The Pan American Health Organization (PAHO) reported fewer than 1 million suspected or confirmed Zika virus cases, whereas this study estimates that 132.3 million people living in the Americas were infected. 

These researchers used a hierarchical Bayesian model with empirically-informed priors, leveraged multiple types of Zika case data from 15 countries to estimate sub-national reporting probabilities and infection attack rates (IARs). 

Zika IAR estimates ranged from 0.084 (95% CrI: 0.067–0.096) in Peru to 0.361 (95% CrI: 0.214–0.514) in Ecuador, with significant subnational variability in every country. 

Totaling infection estimates across these and 33 other countries and territories, our results suggest that 132.3 million (95% CrI: 111.3-170.2 million) people in the Americas had been infected by the end of 2018. 

These estimates represent the most extensive attempt to determine the size of the Zika epidemic in the Americas, offering a baseline for assessing the risk of future Zika epidemics in this region.

The Zika virus (ZIKV) epidemic generated a large amount of concern in the public health community and the general public, leading to a declaration of a Public Health Emergency of International Concern by the World Health Organization (WHO) in February 2016, because of the discovery of a link between ZIKV infection in pregnant women and congenital Zika syndrome (CZS) in newborns.

ZIKV infection is also associated with rare but serious neurological disorders, particularly Guillan-Barré syndrome.

The estimated reporting probabilities for microcephaly in each territory were significantly lower than the reported risk of microcephaly or CZS during pregnancy from several different studies.

The current status of population-level immunity throughout the Americas has important implications for the future of Zika epidemiology in the region. These IAR estimates suggest that a number of areas exceeded the herd immunity threshold, which has also been suggested by other studies.

However, the high degree of heterogeneity in subnational IARs suggests that certain areas in Central and South America that are suitable for ZIKV transmission may still contain a considerable number of susceptible individuals.

In summary, these researchers stated: ‘our results indicate that fewer than 1% of ZIKV infections in the Americas were reported during the recent epidemic.’ 

‘In the absence of detailed seroprevalence data, our subnational and national IAR estimates are an important first attempt at assessing current levels of ZIKV immunity throughout the Americas. Current levels of immunity are a critical determinant of the probability of further Zika outbreaks—and potentially the likelihood of dengue outbreaks as well—in this region in the next decade,’ concluded these researchers.

Underreporting is particularly an issue for pathogens such as ZIKV where the majority of infections are asymptomatic or produce only mild symptoms.

Estimates of ZIKV infections from blood donors in Puerto Rico through 2016 suggest that almost 470,000 people might have been infected in Puerto Rico alone.

Puerto Rico has more administrative level-one areas (78) than the other modeled countries and territories, which suggests that this model may produce overly precise estimates when there is a large sample size and reporting probabilities are too variable between different administrative areas to be reconciled. 

Even with variable reporting probabilities among administrative levels, confirmed cases were underestimated in some administrative areas of Puerto Rico and overestimated in others stated these researchers.

Following the large epidemic from 2015-2017, substantially fewer Zika cases have been reported in 2018 and 2019.

During 2020, the U.S. CDC has confirmed over 70 local Zika virus cases in Puerto Rico.

Because Zika is a cause of severe birth defects, the CDC recommended on September 2, 2020, that pregnant women and couples trying to become pregnant within the next 3-months work with their healthcare providers to carefully consider the risks and possible consequences of travel to areas with risk of Zika.

ZikaNews publishes research-based Zika virus news.