Many rapid test kits cannot tell whether people vaccinated against Covid-19 have indeed built up any immunity to it, the Public Health Ministry admitted yesterday.
Director-General of the Department of Medical Sciences Supakit Sirilak said some of the kits could only test for the virus's nucleocapsid protein but thorough antibody testing required a method that could also test for spike protein. A plaque reduction neutralisation test was among the tests that could measure those antibodies, he said.
"Covid-19 antibody testing must be conducted in labs with biosafety level 3,'' he said.
The doctor added that the results of a spike protein test on Deputy Public Health Minister Sathit Pitutecha conducted 15 days after he received his second Covid-19 shot showed that he had immunity against Covid-19.
The doctor said the minister himself had developed immunity against Covid-19 a fortnight after receiving two doses of the Sinovac vaccine. His antibody level was 115 against the first Covid-19 strains, 85 against the Wuhan strain, 90 against the strain detected in Thailand in January this year and about 40-50 against the G strain currently spreading worldwide.
A study in Chile found that immunity against Covid-19 significantly increased one month after the Sinovac vaccine was administered. The antibody level was just 47.8% 14 days after vaccination but 95.6% after 28 days, it said.
On the virus mutation, the department was working closely with other agencies to monitor British, South African, Brazilian, California, Nigerian and Indian variants, Dr Supakit said.
Meanwhile, Surachoke Tangwiwat, the deputy director-general of the Food and Drug Administration, warned that Covid-19 self-tests could increase the risk of widespread infections if nasal swabs were collected incorrectly because these might result in errors and false negatives.
Dr Surachoke was speaking after it was suggested that the government provide Covid-19 self-tests and that those infected people with mild symptoms should be allowed to stay at home.
The self-tests also had lower accuracy than RT-PCR tests performed by labs, said Dr Surachoke.
"The most important thing is that those with positive results must be hospitalised," he said. "They cannot quarantine at home. They need appropriate and timely treatment. If they do not follow proper guidelines, they might spread the virus without knowing it and cause a new wave of Covid-19 infections."