In this investigation, a confirmed case in a household contact was defined as having received a positive SARS-CoV-2 nucleic acid amplification test result or antigen test result ≤14 days after the index date (date of the index patient's symptom onset or positive SARS-CoV-2 nucleic acid amplification test result or antigen test result), and a probable case in a household contact was defined as the presence of COVID-19-compatible symptoms during the same 14-day period but without a positive SARS-CoV-2 test confirmation. Persons without symptoms and who did not have a positive SARS-CoV-2 test result were not considered to have a case of COVID-19. Analysis of AR among household contacts excluded eight persons with unknown case status (persons for whom it was not known whether COVID-19-compatible symptoms were present and whether SARS-CoV-2 testing had occurred [or if testing occurred, the results were unknown]).

Spectral diffusion in the inhomogeneous paramagnetic resonance lines of Ce 3+ and Er 3+ in CaWO* at concentration ^10 18 spins/cc has been studied by electron spin echo techniques. Measurements show that the spreading of excitation through the line can be approximately described by a diffusion kernel in the form of a Lorentzian function whose width is linear in time. Local field fluctuations due to spin-spin flips and spin-lattice flips appear to be the primary cause of diffusion. In (Ca,Ce)W0 4 below 4.2 °K the lattice time is long, and spin-spin flips predominate, leading to a diffusion rate which is independent of temperature. In (Ca,Er)WC>4 lattice relaxation is more rapid, and the diffusion parameter varies as the lattice time. Nuclei of W 183 in the host crystal have been observed to give a modulation in the envelope of spin echoes, but the corresponding local fields are smaller than those arising from electron spins at the concentrations studied. DIFFUSION IN ELECTRON RESONANCE LINES 2063

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