This study is a multicenter cross-sectional survey-based study conducted in four European countries (Czech Republic, Germany, Slovak Republic, and Turkey). An online questionnaire will be utilized to collect data from volunteer subjects following the STROBE reporting guidelines of cross-sectional studies.

The primary objective of this multicentre cross-sectional survey-based study is to estimate the prevalence of oral side effects of COVID-19 vaccine in the short term.

The secondary objectives are:

• to identify the risk factors of COVID-19 vaccine oral side effects in the short term.
• to evaluate the relationship between oral side effects and other inflammatory side effects of COVID-19 vaccine in the short term.

The target population is healthcare workers as they are identified among the priority groups of COVID-19 vaccine deployment plans in the EU/EEA and the UK. One more reason to select the healthcare workers among different groups of the population, their higher awareness about the possible vaccine-related oral side-effects so that more accurate and reliable results can be obtained.

A self-administered questionnaire will be developed, and its content validity will be tested using a panel of experts. The test re-test reliability of the questionnaire will be tested by 30 volunteers (10 Czech, 10 Turkish, 10 German) to achieve at least a Cronbach's alpha of 0.70. The questionnaire will be available in five languages; Czech, English, German, Slovak and Turkish. Dual forward translation and expert panels will produce equivalent Czech, German, Slovak and Turkish versions of the questionnaire.

The questionnaire will be composed of multiple-choice items divided into four main categories; a) demographic data including gender, age, location of practice, profession, and experience, b) medical anamnesis including medical comorbidities, medications, and recent oral symptoms, c) COVID-19 related anamnesis including vaccination date, previous infection, and exposure to infected cases, and d) vaccine side effects including inflammatory symptoms, cutaneous symptoms, and oral symptoms. (Annex 1)

Inclusion Criteria:

• Healthcare workers who received COVID-19 vaccine during the last 30 days.
• Participating subjects should be at least 18-year-old and able to give their informed consent independently.

Exclusion Criteria:

• The healthcare workers who did not receive the COVID-19 vaccine recently.
• Non-healthcare workers who received the COVID-19 vaccine recently.

The target population is healthcare workers as they are identified among the priority groups of COVID-19 vaccine deployment plans in the EU/EEA and the UK. One more reason to select the healthcare workers among different groups of the population, their higher awareness about the possible vaccine-related oral side-effects so that more accurate and reliable results can be obtained.

In accordance with the General Data Protection Regulation (GDPR), the data controller is Masaryk University (MUNI) and will be used solely by MUNI for the purpose of research in area of public health. The collected data will not be accessed or used by other institutions or for any other purposes. The data will be processed and analysed during the duration of the project (approximately one year). After the project is finished, the data will be encrypted and safely stored by MU as a measure to secure the integrity of the research.