Adults 20-49 key to last year's US COVID-19 resurgence, study says
Adults 20 to 49 years old may be sustaining the resurgence of COVID-19, according to a study published late last week in Science.
The researchers looked at age-specific mobility trends from more than 10 million Americans and used mathematical modeling to estimate the proportion of COVID-19 cases per age-group, finding that young adults less than 35 appeared to have comparable mobility compared with older age-groups as pandemic restrictions eased through Oct 29, 2020. However, the estimated viral reproduction number across 38 states, New York City, and the District of Columbia was highest in those 35 to 49 years old (1.39) and 20 to 34 (1.29). Comparatively, for people ages 10 to 19 and 50 to 64, the reproductive values were about 1.
"The primary mechanisms underlying the high reproduction numbers from 20- to 49-year-olds are that at the population level, adults aged 20 to 49 naturally have most contacts with other adults aged 20 and above, who are more susceptible to COVID-19 than younger individuals, paired with increasing mobility trends for these age groups since April 2020," the researchers write. They add that their model also indicated that those in this age-group may have a higher transmission risk per venue visit.
The researchers go on to estimate the number of COVID-19 cases each age group caused, finding that, until mid-August 2020, before most schools reopened, the percent contribution of virus reproduction was mostly from those 35 to 49 (41.1%) and those 20 to 34 (34.7%). After schools opened in the fall, 2,570 observation days revealed a modest shift, with a 38.2% contribution from those 35 to 49 and 34.0% from those 20 to 34.
Yet there is a caveat: As children and teenagers can cause infection in adults who transmit COVID-19 more easily, the researchers say that school openings are associated with an estimated 25.7% increase in COVID-19 infections and a 5.9% increase in related deaths.
"These findings indicate that adults aged 20 to 34 and 35 to 49 continue to be the only age groups that contribute disproportionately to COVID-19 spread relative to their size in the population, and that the impact of school reopening on resurgent COVID-19 is mitigated most effectively by strengthening disease control among adults aged 20 to 49," the researchers write.
Mar 26 Science study
Pre-symptomatic cases tied to substantial portion of COVID-19 spread
Close contacts of symptomatic COVID-19 index cases have a 3.8-fold greater likelihood of getting infected with COVID-19 than if they were exposed to an asymptomatic index case, but pre-symptomatic transmissions accounted for almost 40% of secondary cases, according to a study published late last week in Clinical Infectious Diseases.
The researchers looked at four provinces and one municipality in China, collecting information on transmission events for lab-confirmed cases. From Jan 5 to Apr 7, 2020, 393 symptomatic index cases led to 128 out of 3,136 infections among close contacts (4.1%), while the 185 asymptomatic index cases led to 12 infections out of 1,078 close contacts (1.1%). A shared household and meal sharing were associated with 8.27 and 2.90 increased risks of transmission, respectively, regardless of whether the index case-patient had symptoms.
The researchers also found that 38% of COVID cases stemming from a symptomatic index case occurred before the person developed symptoms. (The researchers acknowledge their attack rates are lower than other reported data, suggesting that different study settings or China's COVID mitigations may be part of the cause.)
Infections from asymptomatic index cases were more likely to be asymptomatic and less severe, according to the study: 1 in 2 close contacts (6/12) were asymptomatic if infected by an asymptomatic index case, compared with 1 in 4 (128/3,136) infected by a symptomatic index patient. Additionally, none of the symptomatic cases from asymptomatic index cases were severe or fatal, whereas 12 of 96 (12.5%) from a symptomatic index patient were.
Mar 27 Clin Infect Dis study
Vaccine-derived polio cases rose sharply in 2020
In an update on circulating vaccine-derived poliovirus type 2 (cVDPV2), the World Health Organization (WHO) said cases and environmental detections increased in 2020 compared with the previous year, and countries in Africa are at the greatest risk for spread or emergence.
In 2020, there were 959 human cases and 411 environmental detections, up from 2019, when there were 336 cases and 173 environmental detections.
For 2020, cases were reported by 27 countries spanning four regions, though 21 were in Africa, where several genetically distinct cVDPV2 outbreaks are under way. In the Middle East, cases in Pakistan have been linked to illnesses in Iran and Tajikistan. And in the Western Pacific region, outbreaks in the Philippines and Malaysia continue.
The WHO said it considers the African region to be at very high risk of spread or emergence, with the Eastern Mediterranean region at high risk and Europe and the Western Pacific at low-to-moderate risk.
Mar 26 WHO cVDPV2 update
In other polio developments, the WHO today said Somalia's circulating vaccine-derived poliovirus type 3 (cVDPV3) outbreak is over, 28 months after it was detected, with no international spread. And Afghanistan today launched its second polio vaccination drive of the year, a 5-day even that targets 9.9 million children younger than 5 years old.
Mar 29 WHO statement on Somalia's cVDPV3 outbreak
Mar 29 WHO statement on Afghanistan polio vaccine drive