First reports of COVID-19 in New York were preceded by uptick in ILI
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Reports of influenza-like-illness increased during early March in areas of New York State with increased COVID-19 diagnoses, despite average rates of laboratory confirmed influenza declining, according to findings in Clinical Infectious Diseases.
“Being able to accurately and quickly track trends in COVID-19 at the local level is critical to responding to this epidemic and understanding the impact of interventions,” Eli S. Rosenberg, PhD, associate professor of epidemiology and biostatistics in the School of Public Health at the University at Albany, State University of New York, told Healio. “This analysis was undertaken as part of our efforts to track where in New York state COVID-19 may be emerging or declining, using multiple data sources.”
Rosenberg and colleagues used data on influenza-like-illness (ILI) from the Outpatient Influenza-like Illness Surveillance Network (ILINet) in New York state, which includes 140 outpatient providers in 49 of 57 New York state counties outside of New York City. Providers in this network share the aggregate number of all-cause outpatient/emergency department visits and total number of visits for patients with ILI. The researchers also collected data on influenza types A and B and SARS-CoV-2, the virus that causes COVID-19, reported by laboratories to the New York State Department of Health electronically. They used this data to track repots of ILI, confirmed influenza and COVID-19 through the first 6 weeks of the ongoing COVID-19 outbreak.
According to Rosenberg, the study revealed increases in persons reporting to outpatient health care settings with ILI in early March, despite diagnoses of influenza declining in February and March.
“These increases in ILI were highest in the regions that subsequently experienced the largest increases in COVID-19 diagnoses and occurred just prior to those increases,” Rosenberg added.
In all four regions of New York state, the average rate of reported, laboratory-confirmed influenza cases per 100,000 people increased from the beginning of the year until early February before decreasing through the end of March. Data from the study demonstrated that, from March 1 through April 4 (weeks 10 through 14), the average daily influenza case rate decreased by 98.9% in the Central region, 98.7% in the Capital region, 98.4% in the Western region and 96.9% in the Metropolitan region. Then, after declining for several weeks in February, ILI increased in three of the regions starting at Week 10 and in all four regions during Week 11 (beginning March 8) and Week 12 (beginning March 15). The largest increases were seen in the Metropolitan and Capital regions between Weeks 10 and 12 (3.5% to 9.3% and 4.4% to 7.1%, respectively), representing a 2.7-fold change in the Metropolitan region and a 1.6-fold change in the Capital region.
COVID-19 was first confirmed in the Metropolitan region on March 2nd and in the other three regions approximately a week later, according to Rosenberg and colleagues. During Weeks 11 and 12 (March 8-21), the slope of daily COVID-19 cases per 100,000 people in each region was 3.6 (Metropolitan), 0.33 (Capital), 0.20 (Central) and 0.17 (Western). From Weeks 13 to 15 (March 22-April 10), the slope of daily COVID-19 cases was lower in all four
regions (Metropolitan, -0.07; Capital, 0.19; Central, -0.02; Western, 0.14). By the end of Week 15, the weekly average rate of new reported COVID-19 cases was 63.3 in the Metropolitan region, 5.5 per 100,000 in the Capital region, 3.1 per 100,000 in the Central region, and 5.7 per 100,000 in the Western region.
“Until the emergence of COVID-19 in early March, ILI and laboratory-confirmed influenza tracked closely together, with both declining. Thereafter, in the New York state region with the highest rates of confirmed COVID-19 (Metropolitan), ILI increased most sharply. In the other three regions, ILI increased ahead of the emergence of COVID-19,” the authors wrote. “This might signal early COVID-19 activity without diagnosis in those regions or increased concern over COVID-19 and care-seeking among people with mild ILI illness.”
Patient visits for ILI decreased after New York state implemented COVID-19 mitigation strategies, Rosenberg added.
“Because not all SARS-CoV-2 infections are diagnosed, it’s important for public health to locally track other measures alongside COVID-19 diagnoses, such as ILI and influenza diagnoses, in order to understand the full burden of illness and to see early warning of changes in the pattern of the epidemic,” Rosenberg concluded.
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