Epidemiology of SARS-CoV-2 and COVID-19

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) causes a highly contagious disease called CO rona VI rus D isease 2019 (COVID-19). The virus was first reported in Wuhan city, Hubei Province of China in December 2019; in less than 3 months it spread globally and was declared a global pandemic by the World Health Organization (WHO) on March 11, 2020. By March 24, 2020, the novel SARS-CoV-2 had been observed in all seven continents and in 222 countries and territories, with 163,788,738 confirmed cases and a mortality rate of 2.1%.

Symptoms of SARS-CoV-2 infection include fever or chills, cough, shortness of breath, fatigue, muscle or body aches, headache, loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, and diarrhea. Severe cases often develop pneumonia, acute respiratory distress syndrome (ARDS), and multiple organ failure.

Less than 2 weeks after the Wuhan Municipal Health Commission announced the detection of the first cases of coronavirus-associated pneumonia, the WHO issued comprehensive technical guidance on how to detect, test, and manage potential cases based on available information. The first official confirmed novel coronavirus case outside of Wuhan was recorded in Thailand on January 13, 2020, and the United States reported its first case on January 21, 2020. Approximately 3 weeks later, on February 11, 2020, the WHO announced that the disease caused by the novel coronavirus would be named COVID-19. Within 5 days, significant transmission of SARS-CoV-2 and increasing cases of COVID-19 were reported in Iran, Italy, and Spain, and the first cases were reported on the African continent on February 25, 2020.

By March 7, 2020, the total number of confirmed COVID-19 cases worldwide had reached over 100,000. The WHO, alarmed at the rapidity of spread and severity of disease, issued a statement calling for “action to stop, contain, control, delay, and reduce the impact of the virus at every opportunity,” and on March 11, 2020, the WHO announced that COVID-19 was officially a “pandemic.” One month later, on April 4, 2020, infections had mushroomed 10-fold to over 1 million cases of COVID-19 worldwide.

The virus has spread in stages known as surges or waves because of their episodic increase in cases, peak, sometimes a plateau, and gradual decline. These waves of viral transmission can be isolated and studied to reveal the details of the spread during these specific periods and determine any precipitating events or related factors. A study conducted with COVID-19 patients before January 20, 2020 found that the average incubation period for the disease was 5.2 days (95% confidence interval [CI], 4.1–7.0) and the 95th percentile was 12.5 days; the number of individuals likely to be infected by each case (the R ₀ ) was estimated to be 2.2. A later review found R ₀ values ranging from 1.4 to 4.6, with a mean of 3.28. The epidemic doubling time was 7.4 days; however, doubling time will vary with the intensity of the mitigation efforts.

The COVID-19 pandemic has been characterized by several waves that differ in size and intensity across countries and continents. A wave is over when the number of new cases per day declines and then plateaus. “Flattening the curve” is a phrase that was commonly used in the early months of the pandemic, when efforts were focused on preventing transmission because health care facilities were in jeopardy of being unable to care for all cases. Flattening the curve means reducing transmission so that the number of daily new cases is reduced and maintained over a longer period. Locales that observed strict social distancing and other mitigation efforts typically observed flattening of their first wave of the pandemic ( Fig. 1.1 ).

In March 2020, Italy, Iran, and New York City were the hardest-hit regions, with daily cases peaking at 6557 new cases per day in Italy, 3186 cases day in Iran, and 21,000 new cases per day in New York City. In April 2020, COVID-19 cases started to spike in Brazil, and by mid-May COVID-19 cases were surging in India. At the same time, several countries, including Spain, Iran, Italy, Denmark, Israel, Germany, New Zealand, and Thailand, began to ease their lockdown restrictions because of reduction in cases. The total number of cases surpassed 5 million globally on May 21, 2020, a number that had doubled to 10 million by the end of June 2020. Globally, COVID-19 deaths had surpassed half a million people by June 28, 2020, according to the Reuters tally, but doubled to over 1 million on September 18, 2020, 191 days after the WHO declared the novel coronavirus outbreak a global pandemic.

In October 2020, the United States entered its third surge of coronavirus cases, the beginning of its deadliest phase yet. The United States accounted for the highest number of cases and deaths in the world at this point, and globally cases surpassed 50 million by early November 2020.

As of May 18, 2021, over 163 million people had tested positive for COVID-19 and over 3.3 million people had died from the disease across the globe. To date, the three countries most affected by COVID-19 infections are the United States (confirmed cases 32,356,034 and 1.8% mortality), India (confirmed cases 23,702,832 and 1.1% mortality), and Brazil (confirmed cases 15,209,990 and 2.8% mortality). Since March 2020, the overall case fatality rate (CFR) was reported to be around 2.1%; as of May 17, 2021, the CFR has decreased to 0.87% ( Fig. 1.2 ).

The most significant factor in controlling the pandemic has been the introduction of COVID-19 vaccines. The COVAX Facility, a system designed to equitably distribute COVID-19 vaccines worldwide, was developed as testing of COVID-19 vaccine candidates was ongoing. By July 15, 2020, the COVAX Facility had been accepted by over 150 countries, representing over 60% of the world’s population. Pfizer/BioNtech’s COVID-19 vaccine was authorized for use in the United Kingdom on December 2, 2020, and the US Food and Drug Administration (FDA) issued emergency use authorization (EUA) for the Pfizer/BioNtech vaccine on December 11, 2020, and EUA for Moderna’s COVID-19 vaccine on December 18, 2020. Immediately after approval for use, these vaccines were being administered to health care workers and other priority groups. At the end of February 2021, the one-dose Johnson & Johnson vaccine was given EUA by the FDA. As of this writing, individuals as young as 5 years old may receive a COVID-19 vaccine in the United States. Early reports find high vaccine effectiveness for the COVID-19 two-dose vaccines.

Because of the propensity of viruses to mutate, new coronavirus variants are already emerging. Currently six coronavirus variants that are causing significant numbers of symptomatic infections have been identified and are variants of interest or variants of concern. The B.1.1.7 variant was first detected in the United Kingdom but has since been reported in the United States (December 2020). The B.1.351 variant was initially detected in South Africa in December 2020 and was identified in the United States at the end of January 2021. The P.1 variant was first detected in travelers from Brazil at an airport in Japan in early January 2021 and found to be circulating in the United States later that month. The B.1.427 and B.1.429 variants were first identified in California in February 2021 and were classified as variants of concern in March 2021. The B.1.617 variants were first detected in December 2020 in the state of Maharashtra, India. They are currently listed as variants of interest. These variants are concerning because of evidence that they may be more contagious, more virulent, less susceptible to treatment, and/or capable of causing reinfections.

In the following sections, the epidemiology of COVID-19 is presented by continent, starting from its occurrence in Asia. Within each continent, selected countries are discussed as examples of different approaches to mitigation and control of the SARS-CoV-2, especially those with high morbidity and mortality rates. The second part of the chapter deals with risk factors for COVID-19.

Asia

China

The first cases of novel coronavirus (SARS-CoV-2)–infected pneumonia occurred in Wuhan, Hubei Province, China, in December 2019. Morbidity was low; 55% of the patients who developed symptoms before the end of December 2019 were related to exposure at Wuhan’s Huanan Wholesale Seafood Market. The first stage of epidemic transmission was primarily a local outbreak among people with a direct contact history at the seafood market. In the second phase, interpersonal and cluster transmission occurred in multiple communities and families in Wuhan in early January. This was the community dissemination stage of the epidemic. During the second half of January, in particular after the January 25, 2020 celebration of the Chinese New Year, there was a remarkable increase in the number of infected patients in affected cities outside Hubei Province. These cases were attributed to population movement in anticipation of the New Year.

An interim report by an independent panel commissioned by the WHO ^, found the initial Chinese response to the new disease to be inadequate. In response, China imposed a city-wide, strictly enforced lockdown of Wuhan that shocked the rest of the world. Nearly 11 million people were quarantined, and face masks and social distancing were mandatory. Hundreds of field hospitals were erected within days to deal with overwhelming cases of COVID-19. Despite the Wuhan lockdown, COVID-19 outbreaks occurred in other major Chinese cities such as Beijing and Shanghai. These were quickly controlled using the same immediate lockdowns and swift mass testing. Entry into China was managed by introducing new restrictions and quarantine control. Although considered to be harsh, these measures eventually proved effective in mitigating the spread of the virus. From January 3, 2020 to May 21, 2021, there have been 105,647 confirmed cases of COVID-19, with 4861 deaths reported to the WHO.

Japan

The first SARS-CoV-2 infection was confirmed in Japan on January 16, 2020, in a person who had returned from Wuhan, China. Multiple cases of COVID-19 were identified by the end of February 2020 throughout Japan. A second outbreak of infection occurred in Japan around mid-March 2020. However, the level of testing for SARS-CoV-2 conducted in Japan during the two initial outbreaks of COVID-19 was lower than in other countries. The total number of cases of COVID-19 reported in Japan is 97,074, with one of the lowest COVID-19 death rates per capita (1.356 deaths/100,000 population).

Vietnam

Between the first case being reported on January 23, 2020 and August 13, 2020, Vietnam had 911 confirmed cases with 21 fatalities (2% case fatality rate). In response, the Vietnamese government took draconian measures to curb the infection rate, including social distancing, locking down businesses, suspending entry of all foreigners from March 2020 until further notice, and closing the border between Vietnam and Cambodia. These measures were effective, and Vietnam reported 6314 confirmed cases and 46 deaths as of May 21, 2021. ^, However, from May 12, 2021 onward, community transmission cases have been reported from Bac Giang, Hung Yen, Hai Duong, Dien Bien, and Hanoi.

Vietnam reported its first case of the coronavirus variant from the United Kingdom on January 2, 2021 and has reinstituted closures of businesses such as movie theaters and sidewalk cafes, schools, and public transportation and banned inbound flights from the United Kingdom and South Africa.

Taiwan

Taiwan had been spared the worst of the pandemic by containing the spread of COVID-19 among its population of 24 million without a complete lockdown. The island recorded 1128 cases and 12 deaths by the end of April 2021. Most of these cases had been imported from travelers. Although Taiwan is currently experiencing a surge, reporting more than 6761 cases and 59 deaths as of May 27, 2021, these numbers are significantly smaller than those in most countries and territories worldwide.

The newest outbreak, causing over 200 daily cases in the week of May 17, 2021, is thought to have started because authorities had relaxed quarantine requirements for airline crew members in mid-April 2021. New social-distancing rules limiting social gatherings, closing some businesses, and tightening border restrictions have been implemented.

India

The first case of COVID-19 in India was reported on January 30, 2020 in an Indian student evacuated from Wuhan, and the first death was reported on March 12, 2020. ^, By August 2020, India had reported several million cases, with more cases among younger people than has been reported in higher-income countries. Deaths were concentrated in 50- to 64-year-olds.

India’s COVID-19 cases started to decline in September 2020, after peaking near 100,000 daily infections. After a plateau, cases began to rise again in March 2021, and the current peak is more than double the previous one, the severity of which has been described by comparison with the previous one, “the second wave has made the last one look like a ripple in a bathtub.” This surge has been attributed to a combination of factors, including the emergence of highly infectious SARS-CoV-2 variants (coronavirus variant B.1.617 was first detected in India in February 2021), a rise in unrestricted social interactions, and low vaccine coverage. In addition, inadequate resources and measures to combat the spread of COVID-19 have caused India to be considered the epicenter of the global COVID-19 pandemic. India now represents 50% of COVID-19 cases and 30% of deaths globally. From January 3, 2020 to May 21, 2021, there have been 26,031,991 confirmed cases of COVID-19 and 291,331 deaths in India.

Nepal

The surge in cases in India caused a critical upsurge in cases in Nepal, with which it shares a border. The cumulative cases in this country of 29 million are more than 513,000, with more than 6300 deaths as of May 23, 2021. The surging 9300 daily COVID-19 cases are overloading Nepal’s fragile health care system, and the country faces severe shortages of medical supplies and vaccines. Thus the pandemic easily crosses borders.

Russia

Russia is among the four countries with the highest number of confirmed COVID-19 cases as of May 11, 2020. The COVID-19 cases in Russia started later than in many neighboring European countries. Russia acted early to reduce importation of the virus by implementing quarantine for passengers arriving from China (January 23, 2020), closure of the land border with China (January 31, 2020), cancellation of most incoming flights from China (February 1, 2020), restrictions on the entrance of non-Russian citizens from China (February 4), and restrictions on access from Iran and South Korea in late February 2020. In the Russian Federation, from January 3, 2020 to May 21, 2021, there have been 4,983,845 confirmed cases of COVID-19, with 117,739 deaths, reported to the WHO.

Iran

Iran was among the first countries to experience large numbers of COVID-19 cases, but the rate of infection slowed after December 2020 ³² as a result of containment measures such as movement restrictions and business closures across the country. After averaging 14,000 daily new cases, after December 1, 2020, the average daily new COVID-19 cases declined 50% to 6000 to 7000 and fatalities declined by approximately 50% and hospitalizations by 40%. Overall, the pandemic has caused 2,865,864 cases and claimed 79,219 lives in Iran.

Europe

France reported the first recognized case of COVID-19 in Europe on January 24, 2020, and within 6 weeks, all 27 countries of the European Union reported cases. Italy was the epicenter for cases early in the pandemic, reporting 62% of cases in Europe during the first week of March 2020. Italian cases and deaths were concentrated in the northern regions of Veneto and Lombardy during the latter part of February, likely as a result of environmental conditions there (see later section). On March 13, 2020, the WHO declared Europe the epicenter of the COVID-19 pandemic with more reported cases and deaths than the rest of the world combined. In a historic move, on March 17, 2020, the European Union closed all its external borders to prevent further spread of the virus. However, SARS-CoV-2 continued to spread quickly throughout the region, so that by the first week of April, Spain reported 21%, Italy 20%, Germany 15%, and France 11% of cases in Europe.

Differences across European countries can be attributed to a number of factors, including differences in population density, population size, the rapidity with which national governments imposed restrictions on movement of residents, availability of personal protective equipment (PPE) and sanitation supplies, and the restrictiveness of and adherence to stay-at-home and quarantine orders. Countries with fewer deaths locked down earlier, had shorter epidemics that peaked earlier, and had smaller populations.

Sweden, for example, stood out among European countries by not imposing restrictions on movement, masking requirements, or crowd sizes and instead relying on herd immunity to contain the epidemic. In addition, testing, contact tracing, source identification, and reporting were limited and considered inadequate. The effect was that Sweden’s cases and deaths (>1 million cases and >14,000 deaths) are higher than those of other Scandinavian countries (Norway 120,000 cases and 781 deaths; Finland 91,000 cases and 929 deaths ) as of May 21, 2021. The United Kingdom has experienced three waves of the pandemic and has reported 4.458 million cases and 127,710 deaths. The first nonclinical trial dose of the Pfizer/BioNTech COVID-19 vaccine was given December 8, 2020 in the United Kingdom, and vaccination began in earnest. Around the same time, a new variant of SARS-CoV-2, B.1.1.7, was beginning to spread, initiating the third surge of cases and the implementation of renewed travel and social distancing restrictions.

As of May 21, 2021, the European Union reported 51,599,270 cases and 1,093,462 deaths; the countries reporting the most deaths were France (5.568 million cases, 108,314 deaths), Italy (4.183 million cases, 125,028 deaths ), and Germany (3.64 million cases, 87,667 deaths ).

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