How much is too much?

Local experts discuss if there is such thing as over-immunisation and the possibility of herd immunity

After the government approved the resumption of the Test & Go programme starting next week signalling a strong desire to restore the country's tourism and overall economy, the Ministry of Public Health also proposed a vaccination programme, focusing on the 10 tourism and high-risk provinces where a fourth Covid-19 jab is strongly encouraged.

During the two-year period of the novel coronavirus outbreak in Thailand, a number of people have received three shots, if not four, against the infectious disease, as well as vaccines for other viruses, such as influenza. But now that the Covid-19 pandemic is far from over as emphasised last week by World Health Organization chief Dr Tedros Adhanom Ghebreyesus, people start wondering how many shots are actually needed and if there is such thing as over-vaccination.

Renowned virologist Dr Yong Poovorawan, head of the Centre of Excellence in Clinical Virology at Chulalongkorn University, posted on Facebook last week that although a vaccine overdose is not likely to happen, excessive shots mean people can suffer with unwanted side effects.

"It's not necessary for people to get too many [Covid-19] vaccines, be they free or paid ones," he wrote.

Dr Yong made an analogy between receiving vaccines and pouring water into a container. Given scientific studies and the current situation, he said three shots should be enough to fill the immunity container. If the following shots are rushed, it is like the water overflows and goes in vain.

"For those who already received the third shot, it is like their container is full. They can wait until the water evaporates before getting a refill. Pouring water into a container that is full or nearly full does not do any good. People will only have to suffer side effects."

Echoing the same viewpoint on excessive Covid-19 doses is internal medicine specialist Dr Pathumpoom Kooanupong from Paolo Samutprakan Hospital who said that in most cases, the body's immunity level against the virus will decline almost a year after a Covid-19 shot. At that point, a booster is essential. However, for people in Thailand who plan beyond the fourth shot, he suggested a Covid-19 antibody level test prior to getting the fifth shot.

"There is still not much scientific studies and data that explore beyond the fourth Covid-19 shot. Therefore, for those who are uncertain if they should opt for the fifth shot, it's best to check if their immunity level is low and if they require a booster," said Dr Pathumpoom.

In normal circumstances, the development of vaccines is usually a long and complex process that often takes over 10 to 15 years and involves a combination of public and private trials. Covid-19 vaccines are, on the other hand, available across the world under an Emergency Use Authorisation, a mechanism to facilitate the availability and use of medical countermeasures during public health emergencies.

According to Dr Pathumpoom, vaccines for emerging diseases -- including the novel coronavirus infection -- have been developed with a specific purpose to boost people's immunity level as fast as possible in order to quickly keep the spread under control and reduce the public health crisis.

As for the Covid-19 vaccines, instructions regarding the dosage and frequency (duration between each shot) are specified and stipulated by each manufacturer on the basis of studies and data collected from the use of each specific brand.

"Vaccines that have scientific research to back up their efficacy mean those whose manufacturer monitors and follows up the level of immunity once their vaccine is administered," Dr Pathumpoom added. "Most of them only study their own brand because doing that alone already takes a long time and effort. This means vaccine cocktails might not have proven scientific studies to back up their effectiveness.

"As of now, certain brands of vaccines are mixed and used together as a combination only based on expert opinions or small studies in medical schools. There is not yet any cross-research between different brands to seriously explore the effectiveness of vaccine cocktails."

As a result, scientists still fall short of scientific data to officially identify if receiving a Pfizer shot followed by Moderna will be safe or lead to over-vaccination. The same goes with such a claim that over-immunisation is likely to cause the development of malignant cells.

"Some people claim that too much vaccine triggers the white blood cells to overwork and subsequently leads to an abnormal proliferation of cells which can be cancerous. The truth is this is as of now likely to happen only in laboratory test tubes. Studies in lab mice still do not verify such a claim," said Dr Pathumpoom.

The specialist said that although Covid-19 vaccine cocktail recipes implemented in Thailand signify the country's "effort to make the best use of resources available at specific moments", personally he does not recommend mixing too many brands. Completing a proper dosage from one brand before jumping to another is a better way to go.

"I would recommend, say, two shots of Sinovac followed by two shots of AstraZeneca instead of two Sinovac shots followed by one AstraZeneca and one Pfizer shot because after a complete dosage, people will receive the level of immunity as properly studied or researched. More importantly, they will know for sure when they should start the next shot based on reference data from vaccine manufacturers."

Dr Yong also suggested on Facebook that the fourth Covid-19 shot should be at least three to six months away from the preceding shot. Especially after an mRNA vaccine that promises a high level of immunity, people should have a duration of no less than six months before getting the next jab.

For the Covid-19 crisis to end, Dr Pathumpoom believes vaccines are still key. Similar to all infectious diseases in the past, the novel coronavirus pandemic can come to an end only if the world population is immune. And immunity, he added, should come from vaccines instead of having people exposed to the infected. Otherwise, countries will still be in a critical situation where the number of severe cases and fatalities only continues to rise.

"Herd immunity can never be achieved unless people are vaccinated first. If you let people get exposed to the virus and call that a natural immunity, the world will see a great loss and the public health system will eventually collapse. So for herd immunity to work, people require vaccines which serve as a foundation to lower fatality rates and the severity of sickness. Then after vaccines, they can get naturally boosted from small volumes of virus in the environment. That is how herd immunity occurs. We will be safe if others are safe too," Dr Pathumpoom concluded.

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