1. Introduction
With the availability of Covid-19 vaccinations [
1,
2], hospital operations management has faced worldwide the new situation that a vaccine rollout scheme for hospital staff had to be implemented. Due to scarcity of vaccine doses, prioritization decisions have to be made during this rollout. This study makes a contribution to this topic by investigating consequences from different rollout schemes. The study’s focus is the maximization of open hospital capacity which is assumed to maximize patient benefit.
Research studies from the pre-Corona age exist. Their focus was to prove benefit from vaccination policies [
3,
4,
5]. Shortage of vaccine supply was not a particular focus topic; it was known e.g., for yellow fever [
6]. Instead, rather an oversupply and mandatory vaccination were discussed [
7]. The early 2020 vaccination framework of the World Health Organization (WHO) already included aspects of prioritization decisions for a vaccination rollout against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [
8]. The general logistics of the large scale Covid-19 vaccine rollout currently taking place require vaccine doses, vaccinating staff administering the doses, and a process to assign them to patients [
9]. Rollout recommendations or deployment plans have been developed and published by state agencies [
10,
11], also with details on the process of vaccine recommendation decisions [
12].
There is a general agreement that vaccines should be used to their best potential to curb the pandemic’s consequences, in particular during the early vaccine rollout phase when shortage of vaccine supply prohibits an immediate full rollout to the entire population. Decision making under shortage of resources is known from other important medical supplies in connection with the current pandemic [
13,
14]. Insights into prioritization decisions and their consequences have been published for studies considering the entire population [
15,
16,
17,
18,
19]. Delaying a second dose if required by the drug regime has been discussed as an option to reduce vaccine shortage [
20,
21]. In spite of the available material and dedicated rollout recommendations, e.g., [
10,
11], decision making about rollouts to hospital staff can still prove to be controversial. The weighing in of factors such as hierarchical importance [
22,
23] or student status [
24] can lead to differences in the rollout scheme and prioritization among hospital staff. The rollout in the United Kingdom so far has been successful [
25]. Subgroups of a population such as those economically worse off [
26] or minorities [
27] can require special attention, however. In addition, nursing staff, who by the nature of their work are in close contact to patients, must not be forgotten in rollout schemes and prioritization [
28].
In this presented study here, a model hospital is simulated and its capacity is calculated. The hospital’s capacity is limited by available staff. Staff are exposed to the pathogen and are vaccinated after two different rollout schemes which are compared to each other. The first rollout scheme assigns vaccine doses in
hierarchical order top down. It relates to arguments made in the past [
22,
23] that higher-ranking staff are fewer in number, more important as individuals for the functioning of the hospital, and that they are of greater age, which can increase the transmission probability of a pathogen. The alternative rollout scheme first prioritizes between hospital units by
exposure to the pathogen. Accidents & emergencies (A&E), where exposure to nontested outpatients occurs, is assigned vaccine doses first and hospital wards follow behind. On each unit, vaccines are assigned in hierarchical order.
The model is implemented as Visual Basic for Application (VBA) macro. The macro and its embedded version in Microsoft-Excel (Microsoft Corporation, Redmond, WA, USA) are available as open access supplement 1 and 2 under the GNU’s Not Unix (GNU) General Public License version 3, or any later version. We hope that the open access macro code (
Supplementary Materials) will further exchange, and will make accessibility to the study’s work easier. A similar project with results for an entire population, split by age groups, is published in [
18], and pre-Corona work about hospital staff in [
3,
4,
7]. The study’s methodology uses an algorithm similar to known work from hospital capacity planning [
29]. Staff and vaccines are modelled as streams, similar to [
30]. It applies novel insights into the epidemiological spreading of SARS-CoV-2 [
31,
32], and factors influencing disease spreading such as age [
33,
34].
The study’s parameters are set to values typical for the current SARS-CoV-2. However, the methodology and the implemented model are equally applicable to other infectious diseases. The simulated hospital and affected staff are specified in
Section 2.1 and
Section 2.2. Staff structure is a pyramid. This hierarchy form is found worldwide as typical human resources (HR) structure in hospitals. Staff further up in the hierarchy is of greater age. Larger simulations are possible in the future. The investigated effects are independent of hospital size. In
Section 4, we discuss observable trends and predictions for future scenarios.
Although mostly gone unnoticed in Europe and North America, the list of major infectious disease outbreaks in e.g., Hong Kong during the 25 years before the ongoing SARS-CoV-2 pandemic included avian flu H5N1 in 1997, SARS in 2003, swine flu 2009, and avian flu H7N9 in 2013 [
35]. Hence, a vaccine rollout in the general population and amongst hospital staff against an infectious disease might be needed again in the future.
2. Methodology and Model Implementation
For the present study, a hypothetical hospital is modelled as it can be found around the world. Staff on higher hierarchy levels are fewer in number, of greater age, and of greater individual importance for the functioning of the hospital organization. Influence of model parameters is discussed in the results section and in the conclusions section.
2.1. Hospital Structure and Staff Reserve
The hospital is structured in four units. Staff is coming to work every day and is simulated for a variation of initially available staff reserve. Staff size and staff requirements are chosen to reflect a typical hospital situation.
Hospital staff. The hospital staff consists of doctors and nurses. Nurses are not substructured further. Doctors are each assigned to the rank of physician, senior doctor, executive senior, or chief. The model assumes that any staff member is qualified to work on any hospital unit.
Table 1 summarizes the staff numbers initially available at
t = 0 for the simulated base case and the scenarios of staff reserve variation.
Hospital units. The hospital is structured in four units. It is assumed that the hospital operates under the specialty of general internal medicine. A&E receives outpatients. In-patients are treated on wards 1–3. The daily numbers of staff required for 100%/50% operations of each unit are shown in
Table 2. Nurses and physicians are required to be present. The director, executive seniors, and senior doctors provide background service. The chief and executive seniors are expected during staff shortage also to work in the role of senior doctor; they are downward compatible. The specified staff requirements reflect a typical contemporary hospital organization. The total and relative numbers are chosen to imitate a general medicine or surgery clinic.
Hospital capacity Γ is calculated as average of the open status of A&E, and ward 1–3 in Equation (1).
Γ: relative open hospital capacity, γ: hospital unit’s open status, .
2.2. Age Distribution
It is known that there can be an age factor in the epidemiology of a pathogen. SARS-CoV-2 is no exception in that regard [
33,
34]. For each staff member, we assign an age based on a uniform distribution.
Table 3 gives the bounds of the distribution for the different staff roles. The impact of age on infection events is discussed further in
Section 2.4. Age boundaries vary between different countries; e.g., there is no global uniform retirement age. The chosen age distribution reflects a typical hospital setting of our time.
2.3. Vaccination Rollout
The model compares two rollout schemes. In each case, the rollout rate vS defines the number of vaccines per day available to the hospital. Staff are vaccinated, unless they are symptomatic, or already vaccinated. The model is implemented for a vaccine which applies only one dose per person.
Top down vaccine rollout. In the case of the top down rollout scheme, the model assigns doses hierarchically and moves from the top down, starting with the chief. Senior executives, senior doctors, and physicians follow. Nurses are assigned doses last.
Exposure focused vaccine rollout. The exposure focused rollout scheme prioritizes staff by their exposure to the pathogen during their hospital shifts. Due to symptomatic and nonsymptomatic outpatients walking in at A&E, A&E staff’s exposure is assumed greater than exposure of staff on the wards 1–3. A&E staff are vaccinated first; staff on wards 1–3 follow unit by unit in chronological order. On A&E and wards 1–3, again a hierarchical vaccine assignment is implemented.
2.4. Disease Status and Pathogen Transmission
The model calculates each day for each staff member a pathogen transmission probability and the individual disease status.
2.4.1. Staff Member’s Disease Status
The disease status of a staff member can be noninfected, infected and nonsymptomatic, or infected and symptomatic. After pathogen transmission, a staff member is infected, nonsymptomatic and continues working. After the passing of the incubation time
[
36], the staff member becomes symptomatic and remains off duty for the recovery duration of
. Whether reinfections with SARS-CoV-2 are possible and by what probability is being investigated at the moment [
37]. The model assumes that staff can reinfect unless vaccinated. For simplification of disease behavior, staff being vaccinated during
are assumed not to be infected.
2.4.2. Pathogen Transmission
The model assumes that the pathogen transmits from infected human to noninfected human. The model assumes that vaccinated staff are immune to the pathogen and are not infectious for other staff members. For each working day, a probability
is calculated for which a staff member’s status is changed to infected. This probability considers the level of exposure,
as used in epidemiology as reproductive number [
31], the age [
33,
34], and the reduced patient contact of greater hierarchy ranks. Equations (2) and (3) below detail the calculation of the probabilities
and
. Other nonhospital healthcare settings as found in private practice, nursing homes, or at home care services can be simulated alike, based on their specific pathogen exposure patterns.
Pathogen exposure on A&E and wards 1–3. Exposure quantifies in the model the contact to infected humans. On A&E, staff comes into contact with outpatients who walk in and are positive with probability rate . The model assumes that on A&E each staff member has contact with 20 patients during one shift. The model adds the number of infected, nonsymptomatic colleagues on A&E who come to work on that day and increase pathogen exposure to other staff members.
On wards 1–3, outside A&E, the model assumes that all patients are tested and, if positive, isolated so that patients cause no pathogen exposure to staff. The model adds again the number of infected, nonsymptomatic colleagues who come to work on that day to wards 1–3. A base probability of 1/100 is added. By this, the model accounts for the fact that pathogen intake on wards 1–3 vanishes compared to intake on A&E but that it is not nil.
Infection risk Π due to exposure. Infection risk due to exposure is modelled according to research insights about reproduction numbers found in [
31,
32] for SARS-CoV-2. These studies have provided probabilities by which an individual can expect to be infected after a specified exposure event (specified by parameters such as pathogen emission rate, breathing activity, aerosol concentration, or duration of exposure). In this context, the parameter
(pathogen characteristic reproduction number) is used.
is defined as the number of people infected by one index patient. This model now uses the values for
of [
31], multiplies by the number of positive contacts, and divides it by factor five for obtaining the staff member’s personal risk
. The reduction by a factor of five is based on the assumption that the relevant index patient has contact to five members of staff.
Amendment for age and reduced patient contact of greater hierarchy ranks. It is known that social status and age can influence the spreading of infectious diseases [
24,
26,
27]. The model acknowledges this and considers two additional amendment factors, which are both multiplied on the infection risk
.
The age factor
is a number greater or equal 1 and scales linearly between the staff age of 18 and 65, assigned for each staff in
Section 2.2. Values are set to the magnitude as known so far for SARS-CoV-2 [
33,
34].
In modern hospitals, higher hierarchy ranks have typically less patient contact compared to junior doctors or nurses. The model considers this by a factor
which is set to a value greater or equal one. The infection risk for the hospital’s chief and executive seniors is divided by
.
: infection risk,
: positive rate of patients on A&E unit,
: number of positive but nonsymptomatic staff on A&E,
: reproduction number,
: age factor,
: hierarchy factor.
.
2.5. Assignment of Staff to Units and Open/Closed-Status Definition
Staff who is not symptomatic is assigned to work on the hospital’s units. Available staff is by first priority assigned to A&E; followed by wards in ascending ward number. Symptomatic staff is considered off duty.
Open/closed status of a unit is defined for each day by the model according to staff requirements of
Table 2.
2.6. VBA Implementation
The model has been implemented in VBA. It calculates for a period of 100 days the disease spreading under different scenarios. Each scenario is run
n = 500 times to account for statistical unevenness of random number function of e.g., age distribution (
Section 2.2) or pathogen transmission (
Section 2.4.2). The VBA macro is run for this present study in a Microsoft-Excel environment on an Intel(R) Core(TM) i5-6200U central processing unit (CPU) @ 2.30 GHz with 8.00 GB random access memory (Intel Corporation, Santa Clara, CA, USA). Computation time for 500 cycles of each scenario lies at around 25 min, increasing/decreasing with increasing/decreasing staff size.
4. Conclusions and Future Work
The model’s implementation in VBA is iterated for parameter studies. The two rollout schemes for a top down or exposure focused assignment of vaccine doses to staff can be compared in various settings.
In this hospital model, the expected open hospital capacity which is assumed to maximize patient benefit is generally greater for the exposure focused rollout. The model relies on the drawing of random numbers for e.g., age and transmission events. Building statistical averages for
n = 500 model runs ensures convergence of results with a constant,
Figure 2. Standard deviation of open hospital capacity calculated over averages of
n lies below 0.4%.
Results predict an increasing advantage by the exposure focused vaccine rollout scheme under greater disease activity defined by greater pathogen infectiousness and prevalence in patients,
Figure 3. Overall, open hospital capacity can be broken down in open status of hospital units (A&E, wards 1–3). Trends observable for total capacity translate into effects on unit level. Following unit prioritization, predicted loss of unit open status is more pronounced under the top down rollout scheme. In addition, an increasing recovery time leads to greater advantage of the exposure focused rollout,
Figure 5.
The results in
Figure 6 demonstrate the different infection numbers per hierarchy groups. The top down rollout scheme would be most disadvantageous for nurses. Their infection numbers increase greatly under that scheme. In addition, nurses on A&E get vaccinated only after all doctors have been assigned doses. These comparisons between prioritization schemes are of importance. It is known that social status, or minority status influences disease spreading [
24,
26,
27].
Shortage of resources (supply of vaccine doses, or initial staff reserve) decreases the predicted open hospital capacity. The exposure focused rollout scheme leads to better results in those scenarios and is the preferable option when compared to the top down rollout scheme,
Figure 7 and
Figure 8. The influence of greater pathogen transmission probability by greater age is another argument for the case of the exposure focused rollout scheme. Here again, the role of nursing staff who realistically reach ages of 60 and over must be considered.
The implemented and presented model is a first step to investigate decision making strategies for a novel problem [
22,
23]. The model’s complexity is kept to a minimum. Several other ongoing hospital processes could be implemented in it. Extensions could investigate effects of staff moving between hospitals, only partial immunity of staff after a first vaccine dose, or the fact that tests produce false negative and false positive results. The organizational structure of the hospital could be extended so that it contains several medical specialties; surgery, medicine, radiology, and/or anesthesia. For a hospital’s ability to perform presurgical imaging diagnostics and surgery itself, staffing in radiology and in anesthesia is essential.
The foremost limitation of the simulated model is the small size of the simulated unit. Larger organizations can be expected to have exchange between clinics with pathogen transmission events. This is true for staff who might work on wards shared by clinics; and for patients who receive treatment from more than only one clinic. Disease spreading will be more difficult to simulate. The effects from vaccine rollout schemes will be more difficult to isolate and measure. Nonetheless, observed effects in this study are transferable also to larger organizations. The shown influence on nursing staff, the effect of staff age, or the beneficial effect of greater staff reserves will equally be valid; independently of hospital size. Hospital structure with staff ranks, staff numbers, and staff age reflects a system typical for hospitals worldwide. Changes within the defined staff pyramid will not alter the obtained results; overall results patterns are independent of this. Other healthcare settings can similarly be investigated for optimized vaccine rollout schemes. Healthcare workers in private practice are typically enter into contact with outpatients who can’t be tested in advance before entering the premises. Nursing homes, however, might have the resources to isolate new arriving inpatients for 24–48 h (today the typical waiting time for the SARS-CoV-2 test by polymerase chain reaction). If negative, the new inpatient can be assumed noninfectious, and he or she leaves isolation. Home care services on the other hand will be in touch with patients of unknown disease status.
The mathematical evaluation in the presented model implementation builds averages over time. In a more complex implementation of a more diversified hospital model, integrating open status over time and comparison of areas (closed/open) can increase explanatory power.
The model so far only quantifies impact on unit open status. This could be extended into calculations about number of procedures and number of patients affected. The number of patients treated and remuneration by diagnosis-related groups would allow a quantification of the financial impact on annual business results of the hospital.
The scientific understanding of SARS-CoV-2 behavior is expected to increase. Based on this, SARS-Cov-2 mutants, and other future scenarios, new vaccine rollouts might become necessary. After four major outbreaks of infectious diseases in e.g., Hong Kong alone during the 25 years before the coronavirus pandemic [
35], similar situations might repeat in the future.