Risk for transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) among close contacts of infected persons has not been well estimated. This study evaluates the risk for transmission of SARS-CoV-2 among a prospective cohort of 3410 close contacts in China exposed to 391 persons with COVID-19 infection according to different settings of exposure.

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The multiterm fractional differential equation has a wide application in engineering problems. Therefore, we propose a method to solve multiterm variable order fractional differential equation based on the second kind of Chebyshev Polynomial. The main idea of this method is that we derive a kind of operational matrix of variable order fractional derivative for the second kind of Chebyshev Polynomial. With the operational matrices, the equation is transformed into the products of several dependent matrices, which can also be viewed as an algebraic system by making use of the collocation points. By solving the algebraic system, the numerical solution of original equation is acquired. Numerical examples show that only a small number of the second kinds of Chebyshev Polynomials are needed to obtain a satisfactory result, which demonstrates the validity of this method.

The Newtonian gravitational constant G, which is one of the most important fundamental physical constants in nature, plays a significant role in the fields of the theoretical physics, geophysics, astrophysics, and astronomy. Although G was the first physical constant to be introduced in the history of science, it is considered to be one of the most difficult to measure accurately so far. Over the past two decades, eleven precision measurements of gravitational constant have been performed, and the latest recommended value for G published by the Committee on Data for Science and Technology (CODATA) is (6.67408 ± 0.00031) × 10−11 m3kg−1s−2 with a relative uncertainty of 47 parts per million (ppm). This uncertainty is the smallest one compared with the previous CODATA recommended values of G, however, it remains a relatively large uncertainty among other fundamental physical constants. This paper briefly reviews the history of G measurement, and also introduces eleven values of G adopted in the CODATA-2014 after the year 2000 and our latest two values published in 2018 using two independent methods.