The risk of severe illness and mortality from COVID-19 significantly increases with age. As a result, age-stratified modeling for COVID-19 dynamics is the key to study how to reduce hospitalizations and mortality from COVID-19. By taking advantage of network theory, we develop an age-stratified epidemic model for COVID-19 in complex contact networks. Specifically, we present an extension of standard SEIR (susceptible-exposed-infectious-removed) compartmental model, called age-stratified SEAHIR (susceptible-exposed-asymptomatichospitalized-infectious-removed) model, to capture the spread of COVID-19 over multitype random networks with general degree distributions. We derive several key epidemiological metrics and then propose an age-stratified vaccination strategy to decrease the mortality and hospitalizations. Through extensive study, we discover that the outcome of vaccination prioritization depends on the reproduction number R 0 . Specifically, the elderly should be prioritized only when R 0 is relatively high. If ongoing intervention policies, such as universal masking, could suppress R 0 at a relatively low level, prioritizing the high-transmission age group (i.e., adults aged 20-39) is most effective to reduce both mortality and hospitalizations. These conclusions provide useful recommendations for age-based vaccination prioritization for COVID-19.
Voltage-To-Time ConvertersOne of the key building blocks of time-mode circuits is Voltageto-Time Converters (VTC) that map an analog voltage to a time difference variable, i.e., a pulse whose width is linearly proportional to the amplitude of the voltage. Once the input voltage is converted to a time-difference variable, time-mode units such as time amplifiers, time adders, time integrators, time differentiators, and Time-to-Digital Converters (TDC) can be utilized for signal processing. VTCs are typically implemented using a Voltage-Controlled Delay Unit (VCDU) with a reference signal with which the input signal is compared from a timing ring Voltage-Controlled Oscillator (VCO) [7]. The VCDU can be generally categorized into direct VCDU and current-starved VCDU [8][9][10]. The former uses a constant current source to charge a capacitor during the sampling period and a current-steering amplifier to sense the difference between the input voltage and the capacitor voltage [11]. The latter, which is more common in recent designs, adjusts the delay of a current-starve inverter using the sampled input. The time difference between the edge of the reference signal generated by the reference VCO and that of the VCDU is directly proportional to the sampled input and is the time-mode representation of the sampled input voltage. Direct VCDUs offer a high conversion gain and a large dynamic range but suffer from limited bandwidth and high power consumption. Currentstarved VCDUs, on the other hand, features a small conversion gain, a large bandwidth, and consume less power.
Time-To-Digital ConvertersTDCs convert a time-difference variable to a digital code [6,7]. The use of TDCs in nuclear science research dates back to 1970s [12,13].
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