The coronavirus disease 2019 (COVID-19) pandemic has resulted in significant morbidity and mortality worldwide. Community-level immunity, acquired through infection or vaccination, is necessary to control the pandemic as the virus continues to circulate (1). mRNA vaccines encoding a stabilized version of the full-length SARS-CoV-2 Spike protein have been widely administered and clinical trial data demonstrated up to 95% efficacy in preventing symptomatic COVID-19 (2, 3). These mRNA vaccines induce potent humoral immune responses, with neutralizing antibody titers proposed as the major correlate of protection (4-6). Current evidence suggests that circulating antibodies persist for at least 6 months post-vaccination (7), though there is some decay from peak levels achieved after the second dose. This decline from peak antibody levels may be associated with an increase in infections over time compared to the initial months post-vaccination (8, 9). Yet, vaccine-induced immunity remains highly effective at preventing severe disease, hospitalization, and death even at later timepoints when antibody levels may decline (10)(11)(12).Previous research has largely focused on responses early in the course of vaccination, with transcriptional analysis identifying potential links between myeloid cell responses and neutralizing antibodies (13).

We report results from continued timing observations of PSR J0740+6620, a high-mass, 2.8 ms radio pulsar in orbit with a likely ultracool white dwarf companion. Our data set consists of combined pulse arrival-time measurements made with the 100 m Green Bank Telescope and the Canadian Hydrogen Intensity Mapping Experiment telescope. We explore the significance of timing-based phenomena arising from general relativistic dynamics and variations in pulse dispersion. When using various statistical methods, we find that combining ∼1.5 yr of additional, high-cadence timing data with previous measurements confirms and improves on previous estimates of relativistic effects within the PSR J0740+6620 system, with the pulsar mass m p = 2.08 − 0.07 + 0.07 M ⊙ (68.3% credibility) determined by the relativistic Shapiro time delay. For the first time, we measure secular variation in the orbital period and argue that this effect arises from apparent acceleration due to significant transverse motion. After incorporating contributions from Galactic differential rotation and off-plane acceleration in the Galactic potential, we obtain a model-dependent distance of d = 1.14 − 0.15 + 0.17 kpc (68.3% credibility). This improved distance confirms the ultracool nature of the white dwarf companion determined from recent optical observations. We discuss the prospects for future observations with next-generation facilities, which will likely improve the precision on m p for J0740+6620 by an order of magnitude within the next few years.

On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (∼10 −9 Hz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition. In this chapter, we describe how timing observations using SKA1 will contribute to detecting GWs, or can confirm a detection if a first signal already has been identified when SKA1 commences observations. We describe how SKA observations will identify the source(s) of a GW signal, search for anisotropies in the background, improve models of galaxy evolution, test theories of gravity, and characterise the early inspiral phase of a SMBHB system. We describe the impact of the large number of millisecond pulsars to be discovered by the SKA; and the observing cadence, observation durations, and instrumentation required to reach the necessary sensitivity. We describe the noise processes that will influence the achievable precision with the SKA. We assume a long-term timing programme using the SKA1-MID array and consider the implications of modifications to the current design. We describe the possible benefits from observations using SKA1-LOW. Finally, we describe GW detection prospects with SKA1 and SKA2, and end with a description of the expectations of GW astronomy.Advancing Astrophysics with the Square Kilometre Array

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