China has been struggling with its largest COVID-19 outbreaks driven by the SARS-CoV-2 Omicron BA.2 subvariant reported by the Chinese Center for Disease Control and Prevention and its Provincial Centers in press conferences since the first day of March 2022 to the last day of March 2022, with over 100,000 people infected in mainland China within 31 days. Although there were more than a hundred disseminations (independent local outbreaks with cases of infection less than one hundred usually) and several moderate outbreaks before March 2022, China has maintained to its “dynamic zero-COVID strategy” by reducing to zero the number of each active chain of transmission, and it seemed to work well even during the period of the 5th wave driven by Omicron in the world3 from November 2021 to February 2022. These outbreaks, driven by the Omicron BA.2 subvariant, started in Shenzhen and Dongguan, adjacent to Hong Kong, two of the most industrialized cities in southern China, on March 01. Hong Kong had just recorded the most severe attack in February, with
(Lewis, 2022)
close to 900 cases of COVID-19 per 100,000 residents in Hong Kong, the highest level recorded anywhere in the world during the pandemic. The virus swept rapidly across 100 cities in 30 provinces of the country, causing a total of 105,546 people to be infected in a month. Among those, there are ten cities with over 800 people infected, and three of them are the most critical.
China’s “dynamic zero-COVID strategy” has been evolving for a year. More mandatory quarantine and isolation measures were implemented and were said to be precise, being able to track every individual and his or her possibility of contracting the virus on every chain of transmission, to disclose, quarantine, and isolate and classify by symptoms every person of infection with support of the most updated app and a large quantity of fully-trained epidemiological investigators and dedicated doctors. China had already reached its high rate of full vaccination before these large outbreaks. As press conferences of the Department of State reported, over 87% of the population was fully vaccinated by mid-February 2022. The full vaccination rate is 87.45% overall by 05 February and 87.77% by 14 March 2022. This rate of full vaccination is 88.01% in the overall population by 24 March 2022, with 84.36% or 222.72 million of those aged 60 and above. A total of 138.24 million (or 52.36% of) people over 60 years old received booster shots. China’s 7th census published its population as 1.412 billion on 1st November 2020, among them 264 million or 18.70% are aged 60 or above4.
Data in this study cover all daily reported numbers of new PCR-positive cases from March 01 to March 31. All 10 cities with over 800 cases of infection were chosen for this statistical analysis. These 10 cities are located from north to south, and some are close to each other, while some are very far away from each other. All cities followed the same mandatory policy and the same test and diagnosis standards published by the nation. None of the cities, except Shenzhen, implemented all territory lockdowns. Genome analysis revealed that all outbreaks in March were driven by the Omicron BA.2 subvariant according to the report from press conferences by the Department of State and NHC of China. There were enough doctors to diagnose and classify all cases of infection daily except for a single one-day delay in Jilin. Every essential event will be described in the related section to avoid missing possible information relevant to influencing factors of transmission, infection or morbidity. During the March outbreaks, the features of the epidemics are different in many aspects in different cities, and some are contradictory of their morbidity rates.
Large differences in morbidity rates in the main cities
Mainland China reported 103,979 domestic COVID-19 cases of infection by the Omicron BA.2 subvariant from 01 March 2002 to 31 March 2022. There have been obvious differences in morbidity rates between the 10 cities since the start of the March outbreaks. Their morbidity rates vary from 4% to 83%, and the nationwide average rate is 37%. Since Shanghai has the lowest morbidity rate of 4% and the highest number of total cases of infection (36,548), the nationwide average morbidity rate excluding Shanghai rises to 55%. Changchun has the highest, 83%, which is twenty times that of Shanghai (Fig. 1 and Fig. 2)
The reasons why there are such differences are unknown. None of the reasons is related to any of the influencing factors of epidemics known today. There are two pairs of cities that can be compared since two cities of each pair are very close to each other, and both have the same climate, population density and characteristics, diet and living habits, religious background, educational background and medical resources. Shenzhen and Dongguan, both adjacent to Hong Kong, are also adjacent. Both are located on the eastern bank of the Zhujiang Delta, which has a very high population density of 11 to 18 million people on approximately 960-770 square miles of land each (5,6,7). Both being immigrants cities (portion of immigrations being 50% and above in the population), the average ages of their population are relatively young, Shenzhen at 32.5 years and Dongguan at 34 years on 1st November 2020 (8,9).
They have also had almost the same level of transportation of people and cargo back and from Hong Kong. A large quantity of cargo and drivers arrived in Shenzhen and Dongguan with an unknown quantity of virus every day during the surge of cases of infection of Omicron BA.2 in Hong Kong in the early stage of its COVID-19 outbreak in February 2022. When its medical system was brought to breakdown by the epidemic, Hong Kong was unable to admit all COVID-19 patients who needed hospitalization. People wished to escape from the outbreak. The border between Hong Kong and Shenzhen or Dongguan was strictly controlled to allow a certain limited number of travelers across through, which was approximately 10% of that in 2019. Illegal entries from Hong Kong to Shenzhen and Dongguan were reported as many dozens according to news release by Public Security Bureau of Shenzhen. “Hide and seek” follow-up to implement mandatory isolation and PCR tests, because travelers by illegal entry did not wish to be found and medical workers wished to call everybody for tests. Outbreaks started from the end of February in both Shenzhen and Dongguan and resulted in 994 cases of infection in Shenzhen and 1,314 cases of infection in Dongguan in one month. Contradictory figures of the morbidity rate of the two cities appeared from the very beginning, starting from 100% to 92% in Shenzhen and 68% to 0% in Dongguan (Fig. 3, “confirmed” should be replaced by “symptomatic” ). Each of these two outbreaks developed simultaneously on its own trend of morbidity rate, and their rates seemed unlikely to change much until they were close to being controlled by stronger anti-epidemic measures. The difference in their final rates, which was 75% in Shenzhen and 12% in Dongguan by 31th March, was still enormous. Their final morbidity rates were close to their initial rates, although daily rates showed strong fluctuation due to fewer daily new cases. One essential event was a whole area locking down in Shenzhen, including 11 districts, for one week starting from 14 March 2022, which was announced I the evening of 13 March 2022. On 18 March 2022, the Shenzhen authority cancelled the lockdown of 5 districts with “zero active COVID-19 cases”. The final termination of the lockdown started at 00:00 on 21 March 2022.
Dongguan’s outbreak did not follow the trend of expansion of its neighbor Shenzhen and had two extraordinary surges of asymptomatic cases (Fig. 4, the red triangle line). The first wave was from 03 March to 06 March, and the second was from 09 March to 12 March. There were belated prevention measures disclosed later on, but this kind of belated measures usually brought to society occulted transmission between cases of infection that were not diseased or did not consult a doctor, likely with an outcome of a higher morbidity rate. However, this did not happen in Dongguan. Its morbidity rate remained steadily low (fig. 3)
Similar pairs of cities are Changchun and Jilin, which are the first two large cities in Jilin Province in northeastern China and are approximately 111 kilometers apart, with a much larger scale of outbreaks reaching a total number of cases of infection of 20,675 in Changchun and 25,227 in Jilin. Their outbreaks started in Jilin on 03 March 2022 with an unknown source, though a possible source was later reported by local media as from South Korea. Before 03 March, cases of infection had been cleared to zero in Jilin and Changchun for days. Changchun’s outbreaks followed one day after Jilin. Their morbidity rates also differed from the beginning, and their levels seemed steady too, with Changchun’s approximately 85% and Jilin’s approximately 50%. Jilin’s number of daily new cases declined steadily from its peak on 26 March 2022 while Changchun was on its platform. By 31th March 2022, the morbidity rates were 83% in Changchun and 54% in Jilin (Fig. 5).
Morbidity rate did not fluctuate much
Shanghai’s outbreak had a very slow start with 2 cases of infection on 01 March 2022, reaching over 100 cases on 13 March. Rapid expansion of numbers of daily reported new cases of infection appeared, and the number reached nearly 6,000 per day within a period of 17 days (Fig. 6). The total number reached 36,548 on 31 March 2022. The morbidity rates of daily new cases here ranged from 8% to 0% over 24 days of the whole month (31 days) (Fig. 5). The morbidity rate of total cases of infection by 31th March was 4% (Fig. 5). As a city with the most critical number of cases of infection among the three (the other two are Changchun and Jilin), Shanghai’s morbidity rate is the steadiest among all 10 cities with statistical significance.
Rapid spread under strict measures
China managed COVID-19 prevention well by adhering to its “dynamic zero-COVID Policy” until Omicron BA.2 attacked it. The daily confirmed new cases remained no more than 100 before 01 March. There was no easing of strict mandatory quarantine and isolation. However, the rapid expansion of daily reported new infection cases revealed that the BA.2 subvariant could still spread very fast and widely in China, with the fastest in Shanghai (Fig. 6). Current strict prevention measures were not able to stop the rapid expansion of Omicron BA.2. China once again had to face the choice of whether to take a higher economic and social cost to control transmission.
Omicron BA.2 transmission unpredictable
The transmission of Omicron BA.2 is unpredictable, nor is its morbidity rate. Chinese cities are more like isolated islands than those of other countries because of strict prevention measures. This Omicron BA.2 subvariant seems to have developed a different pattern of virulence and transmission in these isolated cities of China. Among the ten cities with outbreaks in March, half of them have very low morbidity rates, including Shanghai (4%), Langfang (8%), Binzhou (10%), Dongguan (12%) and Shengyang (18%). On the other hand, these five cities have a very high percentage of asymptomatic cases in confirmed cases, including Shanghai (96%), Langfang (92%), Binzhou (90%), Dongguan (88%) and Shengyang (82%). Two cities have high morbidity rates: Shenzhen (75%) and Changchun (83%).
There are good aspects of the data, as summarized in the following:
Low severity rate and case fatality rate
Nationwide data of China in March shows a percentage of 0.10% for the ratio of severe cases to symptomatic cases or 0.06% (0.07% in Jilin Province) for the ratio of severe cases to confirmed cases since 12 March 2022 (number of accumulated severe cases dropped from 19 on 01 March to 6 on 12 March due to recovery or converting of hospitalized COVID-19 patients from February). The case fatality rate is less than 1/50,000, as 2 deaths being reported by Jilin on 18 March 2022.
Fewer cases need hospitalization
Only Jilin Province publishes data on the classification of diagnosed COVID-19 patients daily. Jilin’s data shows that 99.30% of the symptomatic cases were diagnosed as mild. A total of 0.61% of the symptomatic cases (or 0.41% of the confirmed cases) are moderate, and 0.10% of those (or 0.07% of the confirmed cases) are severe in Jilin Province. A total of 0.48% of confirmed cases will need hospitalization (following guidance of the national Diagnosis and Treatment Protocol for COVID-19 Patients10), including 0.07% of severe or critical cases. Recovered cases retested positive have not been reported but are estimated to be approximately 1%, close to the figure announced by Shenzhen 3rd People’s Hospital on a news release in March 2022.
Asymptomatic case conversion
The ratios of asymptomatic cases converting to symptomatic cases are 6%, 2.34%, 0.25% and 4.64%, respectively, in Jilin Province (including Changchun and Jilin), nationwide excluding Shanghai, and Shanghai and nationwide including Shanghai, in March 2022. These percentages might indicate the impact to the related actual numbers and ratios in the jurisdictions where such converting data are not reported
Possible contribution to epidemiology
No present mathematical models can tell or predict the features of transmission of Sars-CoV-2 Omicron BA.2 stated above. Rapid epidemic expansion has been observed during the March outbreaks driven by Omicron BA.2 subvariant in China. Some new features of the outbreaks are also revealed by data analysis, including large differences of morbidity rates with steadiness in cities, unpredictable transmission, and low severity rate and low hospitalization need. The study above may bring new thoughts for correction of present epidemiologic theory and mathematical models. It may also give other countries time to be better prepared for the coming 6th wave driven by Omicron BA.2, including preparation and allocation of medical personnel and equipment, cabin laboratories and isolation facilities, and supply of daily necessaries, etc.