A virus expert has warned immune response weakens over time for those who have been vaccinated against Covid-19.

Yong Poovorawan, head of the Centre of Excellence in Clinical Virology of Chulalongkorn University, said in a Facebook post on Monday that a study into recovered patients suggested that immunity against Covid-19 begins to decrease around three to six months after a patient has been infected.

For this reason, individuals who have been fully vaccinated and never infected, as well as patients who have fully recovered from the disease, are likely to need booster shots to maintain sufficient protection against reinfection.

According to Dr Yong, a virus's incubation period plays a significant role in determining the effectiveness of a vaccine, which generally works better against viruses with long incubation periods, such as measles and chicken pox.

However, similar to influenza, Covid-19 has a short incubation period at two to seven days. As such, to maintain immunity against the disease booster shots will be needed, he said.

The expert also warned that those who have been vaccinated, or acquired immunity against Covid-19 as a result of prior infections, can still be reinfected, although the symptoms in subsequent infections tend to be less severe.

"It is highly likely that a third dose of the Covid-19 vaccine will be needed and it is possible that people will need to get vaccinated against the coronavirus periodically," he wrote.

He said that the nation's inoculation programme is a must-do because vaccines are highly effective at preventing people from being hospitalised or possibly dying from an infection.

According to the Centre for Covid-19 Situation Administration, a total of 618,583 people received Covid-19 vaccines between February 28 to April 18. On Sunday, 9,219 people received their first dose, while 843 received their second dose.