Chile Study: The World's First Real-World Study on the Effectiveness of CoronaVac Among Children 3 to 5 Years Old During Omicron Outbreak

On Mar. 15, 2022, a real-world study report accessed the effectiveness of an inactivated SARS-CoV-2 vaccine (CoronaVac) in children 3 to 5 years old in Chile during the prevalence of Omicron was recently published on a pre-print platform Research Square. The results suggested taking two doses of CoronaVac can protect children from COVID-19, including Omicron, lowering the risk of symptomatic cases, hospitalizations, as well as ICU.
This study result from the analysis of a population-based cohort of 490,694 children aged 3–5 years by inverse probability-weighted survival regression models. The study found that 14 days after administered the 2nd dose of CoronaVac, the protective effect against COVID-19 infection was 38.2%, and the protective effectiveness against hospitalization and ICU were 64.6% and 69.0%, respectively. None available data could be used to estimate the protection rate of death due to only 2 death cases being reported during the study period and the model predicted better effectiveness after the third dose of CoronaVac.
This is the world's first published study on the protective effect of the COVID-19 inactivated vaccine in children aged 3-5, and it is also a study on the protective effect of the vaccine for the 3-5 age group after Omicron ravaged the world. Combined with the research results in Chile and the large-scale vaccination data of 260 million doses of CoronaVac in the global population of children and adolescents, it reflects the safety and effectiveness of CoronaVac for minors aged 3 and above. In particular, it is very effective in preventing hospitalization and severe cases caused by infections in children and adolescents.
Another real-world study of 2 million Chilean children and adolescents aged 6-16, conducted by the same expert group, recently published on SSRN also confirmed the effectiveness of the two doses scheme of CoronaVac. The protective effect of infections, hospitalizations, and ICU were 74.5%, 91.0%, and 93.8%, respectively.
Up to now, CoronaVac has been supplied to nearly 60 countries and regions, protecting children and adolescents in 14 countries and regions. SINOVAC will continue to supply vaccines to protect children and adolescents' health globally.

About target population

Since December 6, 2021, Chile has begun to vaccinate local children aged 3 to 5 years with CoronaVac．This study includes all children 3 to 5 years of age affiliated with the national public health insurance program (FONASA, Fondo Nacional de Salud) but excluded children who had probable or confirmed COVID-19 case before December 6, 2021. Among these 490,694 participants, 47626 of them have underlying conditions including asthma, congenital heart disease, etc.
About the prevalence of variants during study period
To monitor and understand the main SARS-CoV-2 variants and lineages in Chile, the Ministry of Health via the national respiratory viruses genomic surveillance program conducted the whole-genome sequencing (WGS) and genotyping of COVID-19 cases. The samples mainly collected from imported cases or community surveillance.  
According to the data between December 22, 2020, and February 21, 2022, there were 70,186 SARS-CoV-2 samples detected with 17.1% of them identified to be Omicron.
Between December 12 and 18, 2021, the number of omicron infected cases accounted for 13.2% of all COVID-19 cases. Since then, the proportion of Omicron cases has continued to increase. From January 30 to February 5, 2022, the ratio was as high as 97.5%. At time of the end of this study, omicron infected cases still accounted for about 90% of all COVID-19 cases. This indicate that the vaccine effectiveness was evaluated during the period of Omicron outbreak.

About the prevalence of variants during study period

To monitor and understand the main SARS-CoV-2 variants and lineages in Chile, the Ministry of Health via the national respiratory viruses genomic surveillance program conducted the whole-genome sequencing (WGS) and genotyping of COVID-19 cases. The samples mainly collected from imported cases or community surveillance.  
According to the data between December 22, 2020, and February 21, 2022, there were 70,186 SARS-CoV-2 samples detected with 17.1% of them identified to be Omicron.
Between December 12 and 18, 2021, the number of omicron infected cases accounted for 13.2% of all COVID-19 cases. Since then, the proportion of Omicron cases has continued to increase. From January 30 to February 5, 2022, the ratio was as high as 97.5%. At time of the end of this study, omicron infected cases still accounted for about 90% of all COVID-19 cases. This indicate that the vaccine effectiveness was evaluated during the period of Omicron outbreak.

Figure S3. Evolution of the predominant SARS-CoV-2 lineages in Chile, according to data shared on GISAID platform, December 22, 2020, to February 24, 2022.

About other studies about vaccine effectiveness against Omicron

Up to date, few studies evaluating the efficacy or effectiveness of COVID-19 vaccines against Omicron variants. A new study published in MMWR showed that  among fully vaccinated children aged 5–11 years, VE against any symptomatic and asymptomatic Omicron infection 14–82 days (the longest interval after dose 2 in this age group) after receipt of dose 2 of the Pfizer-BioNTech vaccine was 31% (95% CI = 9%–48%), adjusted for sociodemographic characteristics, health information, frequency of social contact, mask use, location, and local virus circulation. Among adolescents aged 12–15 years, adjusted VE 14–149 days after dose 2 was 87% (95% CI = 49%–97%) against symptomatic and asymptomatic Delta infection and 59% (95% CI = 22%–79%) against Omicron infection. [1]

About more studies on severity and consequences of children cases

Although previous studies suggested that children and adolescents are at lower risk of hospitalization, ICU admission and death following COVID-19 infection than adults and the elderly, EU CDC surveillance data showed that the number of cases of 12-17 years is higher than other age groups. Symptomatic cases of 12–17 years old population have risen since July 2021 due to  Delta and Omicron variants are widespread in all age groups. In recent months, COVID-19 infection cases and hospitalization cases of children aged 5-11 has been increasing in EU/EEA countries. The rate of hospitalization after infection in children with underlying diseases is 12 times higher than that in healthy children, and the rate of ICU admission is about 19 times higher. [2]
In addition, according to the US CDC, there may be a transient or long-term complications after COVID-19 infection among children. Children may develop serious complications, such as multi-system inflammatory syndrome (MIS-C), a rare but severe disease that can lead to inflammation in different body parts such as the heart and lungs. As of 11 January 2022, more than 2,300 MIS-C cases aged 5-11  have been reported in the United States.[3]

Reference:

[1]https://www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7111e1-H.pdf

[2]https://www.ecdc.europa.eu/en/publications-data/interim-public-health-considerations-covid-19-vaccination-children-aged-5-11  

[3] https://www.cdc.gov/vaccines/covid-19/planning/children/10-things-to-know.html