A Blockchain Approach to Support Vaccination Process in a Country

Andrei Carniel, Gustavo Leme, Juliana de Melo Bezerra and Celso Massaki Hirata

Computer Science Department, ITA, S

ao Jos

e dos Campos, Brazil

Keywords:

Vaccination, Blockchain, Reliability, Vaccine Administration, Ethereum.

Abstract:

Vaccines are important means to prevent diseases and save lives. Data related to vaccination process, such

as vaccine identiﬁcation and number of vaccinated people, is critical to the production and distribution of

vaccines in a way to achieve the desired immunization in a country. We propose a reliable approach based

on Blockchain to manage vaccination data. We describe the roles played in a vaccination process as well

as their relations. Aiming to validate our approach with a prototype, we present the data required in two

scenarios, including the registration of a vaccine administration and the visualization of the vaccination history

of a citizen. We discuss the potential of our proposal, by indicating analysis that can be conducted with the

vaccination data, as well as challenges to be addressed in further investigations.

1 INTRODUCTION

Vaccines are an essential instrument used for life

preservation and protection of people against different

and harmful diseases. Vaccines train the immune sys-

tem to create antibodies without the person come into

contact with the diseases. To get vaccinated is critical

not only to protect the own individual, but also to help

reduce the spread of diseases (WHO, 2021). In many

countries the vaccination process is a human right for

any citizen. A government has concerns about vac-

cination control, distributed batches and immunized

people. There is then a particular way to manage the

vaccination process of population (Brazil, 2020). To

accomplish its goals, a common practice is the cre-

ation of strategic vaccination plans (Anderson et al.,

2012).

However, just having a vaccine is not enough;

traceability is required in order to know production lo-

cation, storage locations, warehouse conditions, man-

ufacturer, and other information. This data provides

a way to ensure vaccine quality and safety in vaccine

supply chains (Yong et al., 2020). With data about

population and vaccination, it is possible to improve

the vaccine management and medical resources (An-

derson et al., 2012).

By analyzing short periods of time, vaccines pre-

vent disease dissemination and decrease the mortality.

When considering long periods of time, it is possi-

ble to identify people that tend to have better health

conditions, high rate of life expectancy and reduced

spends on medical care with a speciﬁc disease that has

a vaccine (Yong et al., 2020). Vaccines have an impor-

tant function to society in the immunization process,

since they aid to eradicate or control diseases that in-

jury people through years, being a cost-effective way

to protect people.

In a pandemic case, a vaccine can be the best

option as a countermeasure. Individuals are in gen-

eral prioritized to receive the vaccine based on their

risk proﬁle and transmission rates. A good strategy

is essential for the success of an immunization pro-

cess (once the disease generally affects more speciﬁc

groups of people), always considering an effective use

of vaccine resources (

Ozaltın et al., 2014). The used

strategy is evaluated with behavioural mathematical

models (Brazil, 2020). Nevertheless, data regarding

vaccination is needed in order to have foundation for

accurate analysis and strategic plans. Obtaining such

data can be a problem to many countries (Palacio

et al., 2017). A common issue is to register vaccines

on a paper of each individual, without any backup

record. The main challenges in vaccination process is

then regarding traceability, lack of information, and

lack of patterns.

In this paper, we describe a reliable approach

to support the vaccination process of a popula-

tion, by registering vaccination data in Blockchain.

Blockchain consists of distributed ledgers, well-know

by the cryptocurrency Bitcoin, but that also has the

attention of many different industries. In Blockchain,

data is distributed across the network, which makes

Carniel, A., Leme, G., Bezerra, J. and Hirata, C.

A Blockchain Approach to Support Vaccination Process in a Country.

DOI: 10.5220/0010520003430350

In Proceedings of the 23rd International Conference on Enterprise Information Systems (ICEIS 2021) - Volume 1, pages 343-350

ISBN: 978-989-758-509-8

343

difﬁcult a data tampering attack. There is also a

consensus mechanism among network nodes to de-

cide which blocks are stored in chains (Malik et al.,

2019)(Nofer et al., 2017). In our proposal, we de-

tail the existing roles in the vaccination process and

their relations. Roles are described considering their

responsibilities in recording and accessing data.

In the next section, related work is presented. In

Section 3, we describe a prototype that implements

the proposal, by focusing on vaccine administration.

In Section 4, there is a discussion about the implica-

tions regarding the proposal. In the last section, we

present concluding remarks and indicate future work.

2 RELATED WORK

Traditional vaccination methods consist of a tech-

nique called number labelling (Yong et al., 2020),

which does not allow accurate vaccine traceability.

Using number labelling, there is only estimated num-

bers of a situation, without a conﬁrmation if these

numbers represent the reality. For instance, a gov-

ernment knows the vaccine doses number per region

and used doses; however, they do not know who is

vaccinated in a given neighbourhood or city.

Vaccination process faces challenges. For in-

stance, records can be forged in order to hide the real

numbers of vaccinated people or vaccines dosages can

be stolen (Yong et al., 2020). In the cases of large

amount of immunizations with no centralized data, it

is faced difﬁculties on data computation (Peng et al.,

2020). With a data tampering attack, it is possible

to lose relevant data (Peng et al., 2020). There can

also be cases in which people travel to another region

to get the vaccine, impacting the resource allocation

(Queiroz et al., 2020). It is important to have a coor-

dination between vaccine transportation and produc-

tion, providing vaccine distribution supervision (Hu

et al., 2019). Furthermore, the lack of registered

records can make more difﬁcult a vaccination cam-

paign (Eisenstadt et al., 2020).

Yong et al. (2020) proposes a work that uses

Blockchain and machine learning in the management

system for the supervision of vaccine supply. The

main concern is to provide a reliable mechanism

where it is possible to trace problems during the vac-

cine production, such as vaccine expiration and vac-

cine record fraud. The mechanism also suggests a

vaccination recommendation, allowing to identify the

better vaccine in a given scenario. Despite of present-

ing an architecture, the authors do not describe roles

or protocols to be adopted by the tool.

Peng et al. (2020) present a work centered on

a safe vaccine production, using Blockchain method

for supervision. The idea is to keep private enterprise

data and public information with product records and

vaccine information (e.g. vaccine name, expiration

date). The work presents an initial application of

Blockchain; however it does not present details of the

roles in the suggested process.

Eisenstadt et al. (2020) describe an approach to

certiﬁcation, using Blockchain and mobile devices.

The work aims to improve the development phase of

an antibody or a vaccine, in order to provide data to

the certiﬁcation process (by preserving privacy). The

authors present brieﬂy the roles when using their tool.

Due to Covid-19, Brazilian government created

the “Conecte SUS” app to have a digital record of

vaccination. It aims to trace the vaccination in pub-

lic and private institutions (Brazil, 2021). The work

describes and discusses usability and scalability of

Blockchain on mobile devices platforms. It provides

an explanation of the vaccination registry using a

smartphone, but the system as a whole is not detailed.

3 A BLOCKCHAIN APPROACH

FOR VACCINATION

Here we describe our digital approach to aid the vac-

cination process in a country. We present a scheme

with roles and their relations, considering the process

since the production of a vaccine until its inocula-

tion. The goal is to address problems related to lack

of registration records and standardization of records,

which in turn can aid the deﬁnition strategic vaccina-

tion plan, as well as the coordination of vaccine pro-

duction and distribution. As a digital platform, we use

of Blockchain mainly due to its reliability character-

istic. So, it is possible to eliminate the lack of records

(since information about the vaccine is registered), to

avoid data tampering and to mitigate cases of forged

records.

It is known that vaccination is a complicated pro-

cess, which can be different in distinct countries. In

this paper, we detail a general and reliable protocol.

Figure 1 shows the proposed approach. Roles are

needed to be deﬁned in a way to control data manip-

ulation in the Blockchain. In order to control data

consistency, we delimit what kind of data each role is

allowed to register or fetch.

Government refers to the entity that controls and

coordinates processes around a society. Government

is the main role and it is allowed to access almost all

data. Person represents any individual in a society. A

Person can receive a vaccine. Enterprise is respon-

sible to develop, test and distribute vaccines. Vac-

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344

Figure 1: Blockchain approach for vaccination with roles and relations.

cination Center is any medical center authorized by

Government to vaccinate people. Vaccination Center

receives and stores vaccines. It has Nurses who can

immunize Person using vaccines. Nurse refers to any

professional allowed to immunize people, who is reg-

istered in a Vaccination Center.

Blockchain represents the network that Govern-

ment, Enterprise and Vaccine Center access and

where data is stored. Data is classiﬁed into public and

private data. The main reason of using Blockchain

technology is because it is a decentralized public

ledger system and it has intrinsic consensus protocols

(Saito and Yamada, 2016). An attack to this network

requires a considerable cost and time, which so dis-

courages the attack. At the same time, by using de-

centralized records, it can be easy to access informa-

tion in a reliable way (Yang et al., 2019).

3.1 Roles in Detail

Here we better specify the roles of our approach, by

describing the expected interactions and permissions.

3.1.1 Government

Government plays a crucial role in assuring veracity

and integrity of data (stored in Blockchain) related to

other roles. The main reason is that governmental en-

tities must regulate processes regarding the approval

of vaccines, the registration of citizens, and the oper-

ation of companies.

We assume that there is only one Government in

this approach, including its various levels of hierar-

chy. Since everyone in society needs to follow rules,

the Government may have access to all the informa-

tion about the vaccine test and composition. It allows

fast analyses and accurate answer in many cases, such

as a pandemic case, or to create strategic vaccinations

plans.

Government accesses the following data: vaccine

to be certiﬁed, vaccine information, vaccine analy-

ses, vaccines stored in a Vaccination Center, and Per-

son’s history. Government has privilege to register

the following data: Person’s identiﬁcation, Vaccina-

tion Centers, Enterprises, strategic plan, certiﬁed vac-

cines, Enterprise inspection, Vaccination Center in-

spection, and Person inspection.

Such information characterizes the core data used

A Blockchain Approach to Support Vaccination Process in a Country

345

by other roles, allowing the system to work properly.

The majority of this information needs to be public, in

a way to provide data visibility to everyone on eligible

legal places. Information regarding Person identiﬁca-

tion needs to be private in order to keep individual

rights of privacy. Moreover, private data is all those

that can be linked and later used to identify a Person,

for instance, the information about who takes a given

vaccine.

3.1.2 Person

Person has its identiﬁcation provided by Government,

which is used to access its personal history. It is ex-

pected to have cases where a given person has more

than one role, for instance every Nurse is also a Per-

son in society.

Person registers only its personal information,

such as telephone and address. It veriﬁes in the sys-

tem all the administrated vaccines in his/her life his-

tory. Person accesses the public information about the

vaccination process, for instance: certiﬁed and stored

vaccines; inspections in Enterprise and Vaccination

Center; and vaccines’ information and analysis. The

goal is to provide means to Person to ﬁnd relevant in-

formation about vaccines and to choose places to be

immunized.

3.1.3 Enterprise

Enterprise is responsible for documentation, research

and production of vaccines. It is expected that a coun-

try has many authorized Enterprises, including na-

tional and foreign ones. Each Enterprise has its pri-

vate data (such as a detailed description of business

rules, and scientiﬁc methods) and contributes to pub-

lic data (such as tests and vaccine information) in

Blockchain.

Regarding the privileges for data registry, Enter-

prise can provide data about vaccines considering dis-

tinct phases as: research, development, test, and cer-

tiﬁcation. The objective here is to give to society

knowledge about the implications of the use of some

vaccines, mainly if a vaccine has side effects on a de-

termined group of people.

For Enterprise, it is speciﬁcally important to ac-

cess Person’s inspection, in a way to support a market

analysis and start a vaccine production. The Enter-

prise’s private data needs to be accessible only by it-

self. In case of audit or vaccine certiﬁcation, Govern-

ment can access some critical data provided by Enter-

prise.

3.1.4 Vaccination Center

Vaccination Center has access to all the Enterprise’s

public data and the Person’s data. Vaccination Center

provides immunization records only of vaccines ap-

proved by Government. Vaccination Center accesses

the Person’s information in order to provide the cor-

rect diagnosis for vaccine application.

Data that Vaccination Center can access includes

the certiﬁed vaccines. Besides, data regarding the

Vaccine Center inspection is useful to provide feed-

back for vaccine usage based on data statistics. As

privileges for data registry, Vaccination Center regis-

ters contracted Nurse and stored vaccines. The infor-

mation about stored vaccines needs to be public, since

it can be valuable to a citizen to search where vaccines

are available.

3.1.5 Nurse

Nurse has access to the vaccine’s public information

and the Person’s information, since Person provides a

identiﬁcation to receive a vaccine. Nurse registers in

the system the vaccines administrated to Person. In

this way, Person has its vaccination history available

at any time.

Nurse is responsible for administrating only the

necessary vaccines, given the provided Person’s his-

tory and diagnosis. As well, Nurse has the compro-

mise to keep all the Person’s data secure, aiming to

preserve privacy. Nurse also reports to Vaccination

Center about the consumed supplies and immunized

people.

3.2 Blockchain Data

In order to describe the needed data to be kept in

Blockchain, we consider a speciﬁc phase of the vac-

cination process: the vaccine administration, which

means when someone is immunized with a vaccine.

The vaccine administration is an essential phase, since

it supports the extraction of accurate information to

make vaccination analysis.

The Blockchain data needed to the vaccine admin-

istration is the following:

1. Patient Identiﬁcation: a record to identify a per-

son. All the person history is linked to this identi-

ﬁcation.

2. Vaccination Center Identiﬁcation: a record to

identify a medical center. With this data, it is pos-

sible to identify the ﬂow and demand for vaccine

supplies.

3. Nurse Identiﬁcation: a record to identify each

nurse.

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4. Vaccine Identiﬁcation: each vaccine needs to

have its identiﬁcation (e.g. vaccine for COVID).

5. Vaccine Manufacturer: identiﬁcation of manu-

facturer, since Enterprise who produces the vac-

cine needs to be speciﬁed.

6. Vaccine Batch: each vaccine has a batch, be-

ing possible to have many batches of the same

vaccine. Enterprises should inspect all vaccines

batches and store their data.

7. Location: place where the person was immu-

nized, since it is necessary to know the correct lo-

cation given that Vaccination Center can change

its address later.

8. Date of Administration: the date when the per-

son received the vaccine.

9. Vaccine Expiration Date: the date in which the

vaccine is still valid.

All data stored in Blockchain network can be used by

approved governmental entities, which can conduct

speciﬁc studies and analysis, in beneﬁt of all roles.

For instance:

• Number of people immunized by a given vaccine:

It provides evidences about preference or usage of

a speciﬁc vaccine type.

• Number and location of people immunized by a

given vaccine: It helps Enterprises to plan vac-

cine production. Besides, it assists Government

and Vaccine Centers to predict new purchases, and

to create distribution plans considering logistics

cost-beneﬁt.

• Number of people, location and vaccine identi-

ﬁcation: It aids the creation of a new vaccina-

tion campaign, the prediction of second pandemic

waves, and the distribution of medical suppliers.

• Person’s identiﬁcation and vaccination history: It

is used to identify vaccines that need to be admin-

istrated to an individual.

Give all data stored in Blockchain, several data com-

binations can be made to extract information and

knowledge regarding vaccination in a country. We

presented only some examples above. The exploita-

tion of other combinations has potential to be ad-

dressed in future research.

4 PROTOTYPE

This section describes an implementation of a pro-

totype that enacts the approach to manage the vac-

cination process. We implemented smart contracts

related to vaccination administration in Ethereum (a

Blockchan platform), using Solidity as programming

language. We used Remix (an Ethereum IDE) to im-

plement and deploy the smart contracts on a private

local Blockchain.

The private local Blockchain was supported by

Docker containers. Docker containers are considered

due to their easy usage and possibility of project scal-

ability, being more practical than working with virtual

machines. The Blockchain network has three mining

nodes. There was also a third party service for net-

work monitoring, named as Ethereum Network Sta-

tus (shown in Figure 2). The mining nodes were ac-

cessed through a Remote Procedure Call (RPC) in-

terface exposed to the host by the service containers.

The Ethereum Network Status was exposed through

the HTTP protocol on a host port.

The front-end layer was based on Next.js (a Re-

act Framework) and used the smart contracts exposed

by the Blockchain network through RPC, in a way to

provide data storage and retrieval. The prototype has

mainly two functionalities: to register a vaccine ad-

ministrated to a patient, and to provide visibility of a

patient’s vaccination history.

Figure 3 illustrates the usage of the vaccination

registration form, responsible to input data into the

Blockchain when a given vaccine is administrated to

a given individual. The following ﬁelds are required:

patient name (as ‘James Miller’), vaccination location

(as ‘297 Grant Avenue, Auburn NY 13021’), vaccina-

tion center (as ‘IVI - International Vaccine Institute’),

nurse (as ‘Patricia Williams’), and vaccine identiﬁca-

tion (as ‘AZD122-Pﬁzer’).

Through the front-end application, it is also possi-

ble to check all vaccination history of a given patient,

which is illustrated in Figure 4. It is shown the patient

information: individual ID in the country (as ‘324-53-

9801’), and patient name (as ‘James Miller’). Later,

there is a table for each received vaccine. In this ﬁg-

ure, there is one vaccine with the following informa-

tion: type (as ‘AZD1222’), manufacturer (as ‘Pﬁzer’),

batch (as ‘100’), expiration date, application date, ap-

plication location (as ‘297 Grant Avenue, Auburn NY

13021’). There is also data about the vaccination cen-

ter related to that vaccination, including: name of the

center (as ‘IVI - International Vaccine Institute’) and

nurse identiﬁcation (as ‘Patricia Williams’).

5 DISCUSSION

Using Blockchain to support the registration of vacci-

nation data, we propose a reliable approach to coordi-

nate vaccination. It can solve or mitigate problems in

A Blockchain Approach to Support Vaccination Process in a Country

347

Figure 2: Ethereum Network Status monitoring the application.

Figure 3: Form to registry a vaccine administration.

the entire vaccination cycle, mainly related to the lack

of data. All vaccines of a given person are available

to improve diagnosis in cases of a disease.

All stored data can be used by specialists to create

strategic vaccination plans to better assist population,

using only the required resources. By controlling in-

ventories based on vaccine expiration date and asso-

ciating it with vaccines’ distribution, it is possible to

avoid wastes. In cases of problems in a vaccine batch,

Government or Enterprises can coordinate the detec-

tion and removal of such a vaccine; while people who

took the vaccine can be identiﬁed and warned.

Following the proposal, new opportunities related

to vaccination arise. Supposing that a certiﬁed vac-

cine has a recent study that unfortunately shows that it

can be nocuous to children, so such a vaccine needs to

be suspended. With a Blockchain system, the vaccine

certiﬁcation is revoked by Government and all roles

have the updated information about the vaccine, for

instance the system can alert Vaccination Centers that

they are not allowed to use this vaccine any longer to

that speciﬁc public.

As limitations of the proposed approach, we high-

light the need of cooperation among partners, includ-

ing Government, Enterprises and Vaccination Centers

to implement and maintain this service correctly.

Distinct strategies can be established to add peo-

ple in the Blockchain system. Hospitals can register

newborns and their initial vaccines just when they are

born. After that, their proﬁles are updated in a regular

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348

Figure 4: The vaccination history of a patient.

way when they need further vaccines. Other citizens

can be included by Government and requested to go

to a Vaccination Center with their paper vaccination

cards in order to update the system. Other possibil-

ity for unregistered citizens is to have their vaccines

computed in the system only when they take a new

vaccine.

As a future implication, if a person needs to travel

to other country that requires a vaccine inspection,

he/she can retrieve his/her vaccination identiﬁcation

from the system, since it is already certiﬁed by Gov-

ernment. Other possibility is to include in the system

a speciﬁc role to other countries or embassies, so they

can verify in the system the vaccination history of the

traveller. In the same way, any person (with his/her

vaccines records) can verify if it is possible to go to

a speciﬁc country, considering that the countries’ en-

trance requirements are already deﬁned in the system.

6 CONCLUSIONS

We propose an approach, based on Blockchain, to

store and maintain data around the entire vaccination

process. The main goal of the approach is to provide

correct and updated data for aiding citizens (for in-

stance, in cases of disease diagnosis or vaccine in-

spection in a travel) and for supporting institutes (for

example, with the creation of strategic plans for vac-

cine distribution and population immunization).

Blockchain provides a reliable structure for the

approach due to its proof-tamper characteristic, so

data can not be corrupted, and also the entire track of

data is available. In the approach, we describe roles

in the vaccination process and their relations and priv-

ileges. Considering the vaccine administration (the

procedure of registering a vaccine administration in

a person), we present the required Blockchain data,

including patient identiﬁcation and vaccine informa-

tion.

Aiming to validate our approach, we developed

a prototype using Ethereum Blockchain. We imple-

mented a contract to register a administered vaccine

and other to access patient history with all received

vaccines. We understand that, depending on the de-

sired functionality or analysis, other data can be con-

sidered in the approach. For future work, we intend to

specify other scenarios related to vaccination in order

to improve the deﬁnition of the required data. Inves-

tigations need to be conducted with existent institutes

(such as medical centers and enterprises) in order to

identify their expectations and needs, to support more

relevant analysis.

An additional study should consider data privacy

and authorization, which will provide details to clas-

sify data and reﬁne roles’ privileges. Other important

work is to create a basis to store information regarding

the research and development process of a vaccine,

A Blockchain Approach to Support Vaccination Process in a Country

349

which is essential in the certiﬁcation process. Eval-

uation with real users playing all the proposed roles

is also of interest. The deﬁnition and development of

the entire approach is a challenge, however the ex-

pected beneﬁts are enormous, arising then interesting

opportunities of future work.

REFERENCES

Anderson, R., Lloyd, J., and Newland, S. (2012). Soft-

ware for national level vaccine cold chain manage-

ment. pages 190–199.

Brazil (2020). Minist

erio da sa

ude prepara

estrat

egia de vacinac¸

ao contra a covid-

19, website: https://www.gov.br/saude/pt-

br/assuntos/noticias/ministerio-da-saude-prepara-

estrategia-de-vacinacao-contra-a-covid-19.

Brazil (2021). Aplicativo conecte sus: o controle da

vacinac¸

ao contra a covid-19 na palma da m

ao; saiba

como usar, website: https://www.gov.br/saude/pt-

br/assuntos/noticias/aplicativo-conecte-sus-o-

controle-da-vacinacao-contra-a-covid-19-na-palma-

da-mao-saiba-como-usar.

Eisenstadt, M., Ramachandran, M., Chowdhury, N., Third,

A., and Domingue, J. (2020). Covid-19 antibody

test/vaccination certiﬁcation: There’s an app for that.

IEEE Open Journal of Engineering in Medicine and

Biology, 1:148–155.

Hu, X., Peng, S., Long, C., Jiang, H., and Wei, L.

(2019). vguard: A spatiotemporal efﬁciency supervi-

sion method for vaccine production based on double-

level blockchain. pages 1037–1042.

Malik, S., Dedeoglu, V., Kanhere, S. S., and Jurdak, R.

(2019). Trustchain: Trust management in blockchain

and iot supported supply chains. pages 184–193.

Nofer, M., Gomber, P., Hinz, O., and Schiereck, D. (2017).

Blockchain. Business & Information Systems Engi-

neering, 59(3):183–187.

Ozaltın, O. Y., Dalgıc¸,

O. O., and Erenay, F. S. (2014). Opti-

mal distribution of the inﬂuenza vaccine. pages 1411–

1420.

Palacio, M. G., Palacio, L. G., Montealegre, J. J. Q., Pab

on,

H. J. O., Del Risco, M. A. L., Rold

an, D., Salgar-

riaga, S., V

asquez, P., Hern

andez, S., and Mart

ınez, C.

(2017). A novel ubiquitous system to monitor medic-

inal cold chains in transportation. pages 1–6.

Peng, S., Hu, X., Zhang, J., Xie, X., Long, C., Tian, Z., and

Jiang, H. (2020). An efﬁcient double-layer blockchain

method for vaccine production supervision. IEEE

Transactions on NanoBioscience, 19(3):579–587.

Queiroz, M. M., Ivanov, D., Dolgui, A., and Wamba, S. F.

(2020). Impacts of epidemic outbreaks on supply

chains: mapping a research agenda amid the covid-19

pandemic through a structured literature review. An-

nals of Operations Research, pages 1–38.

Saito, K. and Yamada, H. (2016). What’s so different about

blockchain?—blockchain is a probabilistic state ma-

chine. pages 168–175.

WHO (2021). Vaccines and immunization: What is

vaccination?, website: https://www.who.int/news-

room/q-a-detail/vaccines-and-immunization-what-is-

vaccination.

Yang, X., Chen, Y., and Chen, X. (2019). Effective scheme

against 51% attack on proof-of-work blockchain with

history weighted information. pages 261–265.

Yong, B., Shen, J., Liu, X., Li, F., Chen, H., and Zhou,

Q. (2020). An intelligent blockchain-based system

for safe vaccine supply and supervision. International

Journal of Information Management, 52:102024.

ICEIS 2021 - 23rd International Conference on Enterprise Information Systems

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