This study aims to examine the association between gut microbiota composition and the magnitude and duration of immune response in subjects who have received different COVID-19 vaccines in Hong Kong and to identify the differences compared to those COVID-19 recovered subjects.

SARS-CoV-2, the cause of COVID-19, emerged as a new zoonotic pathogen of humans at the end of 2019 and rapidly developed into a global pandemic in March 2020. Over 106 million COVID-19 cases including 2.3 million deaths have been reported to the WHO as of February 9th 2021. Overall 80% of cases are mild/moderate or asymptomatic while 20% may develop severe or critical disease with respiratory failure requiring oxygen support and intensive care. As of 16 February 2021, 10789 cases have been confirmed in HK with 193 deaths while 10231 patients have been discharged from acute public hospitals with a mean length of stay of 15 days. Up to 16 Feb., 2021, there were 568 patients with COVID-19 have been managed at the Prince of Wales Hospital (PWH) with 12 deaths.

Studies showed that the quality of life of the patients who have recovered from COVID-19 was impaired. This situation is similar to some patients who suffered from SARS-CoV1 infection in 2003.

Natural infection by virus triggers an effective system immunity so that the host can resist or highly reduce the chance of re-infection.

In many cases, this protection can maintain a long period of time. On the other hand, vaccination, which aims to induce the similar immunity but in a controlled manner, presents only selected antigen(s) to the host through different delivery strategies. Lacking a normal virus replication cycle, vaccination can only partially mimic the infection process that makes the immunogenicity or durability of the vaccines sometimes not as effective as natural infection. This situation was also observed during the development of SARS and MERS vaccines. A number of studies focused on examining the humoral and cellular immunities from the COVID-19 patients during their acute stage or only few months after recovery. However, the long-term immunities (over a year) from the patients are not yet very clear.

The HKSAR government has procured 2 different types of COVID-19 vaccines for all the HK citizens that is one of the few regions/countries adopt this strategy. BioNTech mRNA vaccination programme will commence in early March 2021 and Sinovac inactivated whole virus vaccine will also be available soon for vaccination after formal approval recently. All these vaccines are so far only evaluated at their own designated population and countries and there is still lack of prospective study to "side-by-side" comparing the microbial biomarkers in the stool of different vaccines. Having different vaccines which will soon be available in Hong Kong, it would be of great interest to investigate and compare the microbial biomarkers in the stool of recovered COVID-19 patients versus those who have received different COVID-19 vaccines by setting up a 2-year longitudinal study.

Inclusion Criteria for Vaccination (VAC) cohort:

1. aged ≥ 18 years old; and
2. have documented to have no previous COVID-19 infection; and
3. will receive any type of COVID-19 vaccines; and
4. agree to hand in study samples (stool and blood); and
5. agree to sign the informed consent

Inclusion Criteria for COVID-19 convalescent (CON) cohort:

1. aged ≥ 18 years old; and
2. recovered from varying severity of COVID-19; and
3. agree to hand in study samples (stool and blood); and
4. agree to sign the informed consent

Exclusion Criteria:

• None

Subjects will be recruited from different clinics in Prince of Wales Hospital, the CUHK Medical Centre, CUHK staff clinics, and community sources. Some subjects will be referred from other observational studies and some will be from any community source.