Though cell size varies between different cells and across species, the nuclear-to-cytoplasmic (N/C) ratio is largely maintained across species and within cell types. A cell maintains a relatively constant N/C ratio by coupling DNA content, nuclear size, and cell size. We explore how cells couple cell division and growth to DNA content. In some cases, cells use DNA as a molecular yardstick to control the availability of cell cycle regulators. In other cases, DNA sets a limit for biosynthetic capacity. Developmentally programmed variations in the N/C ratio for a given cell type suggest that a specific N/C ratio is required to respond to given physiological demands. Recent observations connecting decreased N/C ratios with cellular senescence indicate that maintaining the proper N/C ratio is essential for proper cellular functioning. Together, these findings suggest a causative, not simply correlative, role for the N/C ratio in regulating cell growth and cell cycle progression. Expected final online publication date for the Annual Review of Genetics, Volume 56 is November 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

As the COVID-19 pandemic continues unabated, many governments and public-health bodies worldwide have ceased to implement concerted measures for limiting viral spread, placing the onus instead on the individual. In this paper, we examine the feasibility of this proposition using an agent-based model to simulate the impact of individual shielding behaviors on reinfection frequency. We derive estimates of heterogeneity in immune protection from a population pharmacokinetic (pop PK) model of antibody kinetics following infection and variation in contact rate based on published estimates. Our results suggest that individuals seeking to opt out of adverse outcomes upon SARS-CoV-2 infection will find it challenging to do so, as large reductions in contact rate are required to reduce the risk of infection. Our findings suggest the importance of a multilayered strategy for those seeking to reduce the risk of infection. This work also suggests the importance of public health interventions such as universal masking in essential venues and air quality standards to ensure individual freedom of choice regarding COVID-19.

As the COVID-19 pandemic drags into its second year, there is hope on the horizon, in the form of SARS-CoV-2 vaccines which promise disease suppression and a return to pre-pandemic normalcy. In this study we critically examine the basis for that hope, using an epidemiological modeling framework to establish the link between vaccine characteristics and effectiveness in bringing an end to this unprecedented public health crisis. Our findings suggest that a return to pre-pandemic social and economic conditions without fully suppressing SARS-CoV-2 will lead to extensive viral spread, resulting in a high disease burden even in the presence of vaccines that reduce risk of infection and mortality. Our modeling points to the feasibility of complete SARS-CoV-2 suppression with high population-level compliance and vaccines that are highly effective at reducing SARS-CoV-2 infection. Notably, vaccine-mediated reduction of transmission is critical for viral suppression, and in order for partially-effective vaccines to play a positive role in SARS-CoV-2 suppression, complementary biomedical interventions and public health measures must be deployed simultaneously.

The ongoing COVID-19 pandemic has placed an unprecedented burden on global health. Crucial for managing this burden, the existing SARS-CoV-2 vaccines have substantially reduced the risk of severe disease and death up to this point. The induction of neutralizing antibodies (nAbs) by these vaccines leads to protection against both infection and severe disease. However, pharmacokinetic (PK) waning and rapid viral evolution degrade neutralizing antibody binding titers, leading to a rapid loss of vaccinal protection against infection occurring on the order of months after vaccination. Additionally, inter-individual heterogeneity in the strength and durability of the vaccine-induced neutralizing response to SARS-CoV-2 can create a further public-health risk by placing a subset of the population at risk. Here we incorporate the heterogeneity in inter-individual response into a pharmacokinetic/ pharmacodynamic (PK/PD) model to project the degree of heterogeneity in immune protection. We extend our model-based approach to examine the impact of evolutionary immune evasion on vaccinal protection. Our findings suggest that viral evolution can be expected to impact the effectiveness of vaccinal protection against severe disease, particularly for individuals with a shorter duration of immune response. One possible solution to immune heterogeneity may be more frequent boosting for individuals with a weaker immune response. We demonstrate a model-based approach to targeted boosting that involves the use of the ECLIA RBD assay to identify individuals whose immune response is insufficient for protection against severe disease. Our work suggests that vaccinal protection against severe disease is not assured and provides a path forward to reducing the risk to immunologically vulnerable individuals.

We have entered a new phase of the ongoing COVID-19 pandemic, as the strategy of relying solely on the current SARS-CoV-2 vaccines to bring the pandemic to an end has become infeasible. In response, public-health authorities in many countries have advocated for a strategy of using the vaccines to limit morbidity and mortality while permitting unchecked SARS-CoV-2 spread ("learning to live with the disease"). The feasibility of this strategy is critically dependent on the infection fatality rate (IFR) of COVID-19. An expectation exists, both in the lay public and in the scientific community, that future waves of the virus will exhibit decreased IFR, either due to viral attenuation or the progressive buildup of immunity. In this work, we examine the basis for that expectation, assessing the impact of virulence on transmission. Our findings suggest that large increases in virulence for SARS-CoV-2 would result in minimal loss of transmission, implying that the IFR may be free to increase or decrease under neutral evolutionary drift. We further examine the effect of changes in the IFR on the steady-state death toll under conditions of endemic COVID-19. Our modeling suggests that endemic SARS-CoV-2 implies vast transmission resulting in yearly US COVID-19 death tolls numbering in the hundreds of thousands under many plausible scenarios, with even modest increases in the IFR leading to an unsustainable mortality burden. Our findings thus highlight the critical importance of enacting a concerted strategy (involving for example global access to vaccines, therapeutics, prophylactics and nonpharmaceutical interventions) to suppress SARS-CoV-2 transmission, thereby reducing the risk of catastrophic outcomes. Our findings also highlight the importance of continued investment in novel biomedical interventions to prevent viral transmission.

While the rapid deployment of SARS-CoV-2 vaccines had a significant impact on the ongoing COVID-19 pandemic, rapid viral immune evasion and waning neutralizing antibody titers have degraded vaccine efficacy. Nevertheless, vaccine manufacturers and public health authorities have a number of levers at their disposal to maximize the benefits of vaccination. Here, we use an agent-based modeling framework coupled with the outputs of a population pharmacokinetic model to examine the impact of boosting frequency and durability of vaccinal response on vaccine efficacy. Our work suggests that repeated dosing at frequent intervals (multiple times a year) may offset the degradation of vaccine efficacy, preserving their utility in managing the ongoing pandemic. Our work relies on assumptions about antibody accumulation and the tolerability of repeated vaccine doses. Given the practical significance of potential improvements in vaccinal utility, clinical research to better understand the effects of repeated vaccination would be highly impactful. These findings are particularly relevant as public health authorities worldwide seek to reduce the frequency of boosters to once a year or less. Our work suggests practical recommendations for vaccine manufacturers and public health authorities and draws attention to the possibility that better outcomes for SARS-CoV-2 public health remain within reach.

The strategy of relying solely on current SARS-CoV-2 vaccines to halt SARS-CoV-2 transmission has proven infeasible. In response, many public-health authorities have advocated for using vaccines to limit mortality while permitting unchecked SARS-CoV-2 spread (“learning to live with the disease”). The feasibility of this strategy critically depends on the infection fatality rate (IFR) of SARS-CoV-2. An expectation exists that the IFR will decrease due to selection against virulence. In this work, we perform a viral fitness estimation to examine the basis for this expectation. Our findings suggest large increases in virulence for SARS-CoV-2 would result in minimal loss of transmissibility, implying that the IFR may vary freely under neutral evolutionary drift. We use an SEIRS model framework to examine the effect of hypothetical changes in the IFR on steady-state death tolls under COVID-19 endemicity. Our modeling suggests that endemic SARS-CoV-2 implies vast transmission resulting in yearly US COVID-19 death tolls numbering in the hundreds of thousands under many plausible scenarios, with even modest increases in the IFR leading to unsustainable mortality burdens. Our findings highlight the importance of enacting a concerted strategy and continued development of biomedical interventions to suppress SARS-CoV-2 transmission and slow its evolution.

As the COVID-19 pandemic drags into its second year, there is hope on the horizon, in the form of SARS-CoV-2 vaccines which promise disease elimination and a return to pre-pandemic normalcy. In this study we critically examine the basis for that hope, using an epidemiological modeling framework to establish the link between vaccine characteristics and effectiveness in bringing an end to this unprecedented public health crisis. Our findings suggest that vaccines that do not prevent infection will allow extensive endemic SARS-CoV-2 spread upon a return to pre-pandemic social and economic conditions. Vaccines that only reduce symptomatic COVID-19 or mortality will fail to mitigate serious COVID-19 mortality risks, particularly in the over-65 population, likely resulting in hundreds of thousands of US deaths on a yearly basis. Our modeling points to the possibility of complete SARS-CoV-2 elimination with high population-level compliance and a vaccine that is highly effective at reducing SARS-CoV-2 infection. Notably, vaccine-mediated reduction of transmission is critical for elimination, and in order for partially-effective vaccines to play a positive role in SARS-CoV-2 elimination, other stackable (complementary) interventions must be deployed simultaneously.