key: cord-0719671-wm48ks4p
authors: Tokuda, Yasuharu; Kuniya, Toshikazu; Shibuya, Kenji
title: Potential impact of alternative vaccination strategies on COVID‐19 cases, hospitalization, and mortality in Japan during 2021–2022
date: 2021-08-26
journal: J Gen Fam Med
DOI: 10.1002/jgf2.493
sha: 8a8e1b705d8e4f65bf854f0fe9d7485f11f76153
doc_id: 719671
cord_uid: wm48ks4p

nan

extension strategy) without compromising expected immunity. 4 One of the major concerns about this strategy is that there is a trade-off between wider vaccine coverage with a first dose and limited effectiveness particularly among the elderly population when we would extend the interval. Alternatively, half a standard dose of the vaccine (half a dose strategy) could also be used without compromising the clinical immune effects. 5 The problem is that this is not officially recommended by the manufactures and regulatory authority. Here, we present simulated results of the COVID-19 epidemic projections using these two alternative vaccine strategies in Japan based on cases, hospitalization, and morality from January 2020 until August 2021. The model mainly considered the spread of Delta variant with greater infectiousness; we assumed the basic reproductive number of 8 (greater than 2 times as infectious as previous variants) based on the data from the US Center for Disease Control and Prevention. 6 A SEIR compartmental model with vaccination was used for the curve fitting by updating the estimation per wave. 7, 8 The infection rate was estimated for each period by fitting the model solution to the data before August 12, 2021. On the estimation, we used the data of daily vaccine doses after April 12, 2021. 9,10 More precisely, we divided the population into elderly and younger populations after that date and assumed that if a susceptible individual is vaccinated, then he/she enters the first vaccinated class. In addition, we considered the second vaccinated class and assumed that the infection prevention effects by the first and second vaccinations are 30% and 60%, respectively, and the onset (symptomatic development) prevention effects by the first and second vaccinations are 40% and 80%, respectively. We, moreover, assumed that the immunity induced by the second vaccination will be decreased to 80% after 6 months and to 50% eventually. cases. When half a dose strategy would be employed, a surge of hospitalization and mortality could be delayed ( Figure 1C,D) . The peak cases of both hospitalization and mortality seem to be like that of the current strategy because of the decreased immunity of the vaccines. However, with a delayed peak provided by half a dose strategy, we could gain precious time interval to prepare and conduct a booster vaccine program to reduce the peaks.

As of August 2021, the delta variant of concern is already spreading throughout Japan, while available vaccines are limited. Flexible vaccination strategies should be considered for delaying infection peaks to obtain time interval to reduce the number of cases, hospitalization, and mortality. One-dose vaccination (i.e., interval extension strategy) for a wider population and/or low-dose vaccination (i.e., half a dose strategy) for many could resolve the current vaccine shortage and suppress a surge requiring lockdown. The rapid implementation of these potentially life-saving strategies should be considered for unprecedented phase that are coming for next several months.

F I G U R E 1 (A) Interval extension strategy. Changing the ratio of vaccine allocation between the first and second doses based on the interval extension strategy starting from September 2021. The simulation assumed three different distribution ratios for the elderly and young: The distribution ratio is 5:5 (elderly:young) in purple, 3:7 in green, and 1:9 in blue curves. (B) Half a dose strategy. The simulation employed half a standard dose of vaccination and thus doubled the pace from the daily pace for the entire population of Japan with vaccination with slightly lower (efficacy proportion = 0.9) or the similar (efficacy proportion = 1) effects, starting from September 2021. The simulation assumed three different distribution ratios for the elderly and young: The distribution ratio is 5:5 (elderly:young) in purple, 3:7 in green, and 1:9 in blue curves. (C) Expected hospitalization cases by half a dose strategy. The simulation employed half a standard dose of vaccination similarly to that of B. (D) Expected death cases by half a dose strategy. The simulation employed half a standard dose of vaccination similarly to that of B

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