COVID-19 may have gone undetected in nearly 20,000 adults in HK

Almost 20,000 individuals, or about 0.3 percent of Hong Kong’s population, may have had undetected coronavirus disease 2019 (COVID-19), according to a serology study of 4,198 adults conducted by the Chinese University of Hong Kong (CUHK).

The study’s participants (aged 18–39 years, 32 percent; aged 40–59 years, 39 percent; aged ≥60 years, 29 percent; female, 60 percent), recruited between 21 April 2020 and 18 April 2021, underwent serology testing for immunoglobulin G (IgG) antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and completed an epidemiological questionnaire.

“Half of the participants had previously tested negative for SARS-CoV-2, while 5 percent had been placed under quarantine and 4 percent had had contact with infected patients,” said Professor Martin Wong of the Jockey Club School of Public Health and Primary Care, CUHK. “In terms of symptom history since November 2019, 67 percent had experienced no respiratory or gastrointestinal [GI] symptoms, 17 percent had had respiratory symptoms, 5 percent had had GI symptoms, 9 percent had had both respiratory and GI symptoms, and 2 percent had had fever.”

“Serology results showed presence of SARS-CoV-2 IgG antibodies in six participants [0.14 percent], including four participants 18–39 years of age and two participants 40–59 years of age,” reported Professor Paul Chan of the Department of Microbiology, CUHK.

“Five of the six participants with SARS-CoV-2 IgG antibodies had previously been placed under quarantine and tested negative for SARS-CoV-2 during the quarantine period. Five of them had experienced respiratory symptoms, and five had a history of travelling to Europe,” he continued.

“Based on this finding and after statistical adjustment, 19,659 adults in Hong Kong, or about 0.3 percent of our total population, are estimated to have had undetected COVID-19,” Chan pointed out.

With 11,828 cases of COVID-19 identified as of 20 May 2021, and 1,756 children and adolescents estimated to have been infected, about 0.44 percent of Hong Kong’s population (n=33,243) are estimated to have had COVID-19.

“This means that more than 99.5 percent of our population need to be vaccinated for protection against COVID-19,” said Chan. “The ratio of confirmed to undetected COVID-19 cases in Hong Kong is approximately 1:2, indicating that our containment and contact tracing strategies are effective in preventing the spread of COVID-19 in the community.”

Both vaccines effective across age groups

In another study, the researchers conducted serology tests in 111 individuals 22–82 years of age (aged ≥70 years, 10 percent; presence of chronic diseases, 14 percent) who had received two doses of either the mRNA (n=48; 43 percent) or inactivated (n=63; 57 percent) COVID-19 vaccine as of May 2021. All study participants tested positive for neutralizing antibodies against SARS-CoV-2.

“This finding suggests that both vaccines are effective in all age groups eligible for vaccination,” said Chan. “Antibody testing after vaccination is not necessary.”

In terms of side effects after the second dose, injection-site discomfort was the most common, reported in 90 percent vs 27 percent of mRNA vs inactivated vaccine recipients. This was followed by fatigue (56 percent vs 16 percent), muscle ache (31 percent vs 5 percent), fever (35 percent vs 2 percent), and headache (27 percent vs 3 percent).