What We Know About a Potential Second Wave of COVID-19

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Revised on 29 September 2020, 12:40 p.m.
Any substantive changes in this HillNote that have been made since the preceding issue are indicated in bold print.

(Disponible en français : Ce que nous savons au sujet d’une possible deuxième vague de COVID-19)

Scientists have warned of a potential second wave of COVID-19 since soon after the World Health Organization declared a global pandemic on 11 March 2020. (COVID-19 is the disease caused by infection of the virus SARS-CoV-2.)

This HillNote provides an overview of the occurrence of second waves in previous epidemics and pandemics, and the evidence that supports the possibility of a second wave during the COVID-19 pandemic.

What is a Second Wave of Disease?

While the term “second wave” has been used frequently in the media to refer to a potential resurgence of COVID-19 cases, in the context of pandemics, the term has no agreed-upon scientific definition.

Canada’s first case of COVID-19 was confirmed on 25 January 2020 and was travel-related. Community transmission was first confirmed in early March 2020. Cases accelerated to peak in mid-April (<2,000 cases per day), then slowly decreased through to the end of June (~200 cases per day).

Canada, at least in certain regions, appears to be experiencing a second wave of the pandemic since the low infection rate that was sustained for months is now beginning to rise steadily rather than occurring in small clusters that are being quickly contained.

A graphical representation of a pandemic with two waves for Canada – the H1N1 influenza pandemic of 2009-2010 – is shown below.

Hospital Admissions and Deaths from H1N1 Influenza in Canada, 2009-2010

Graph shows two waves of H1Ni influenza, the first in late spring 2009 and the second stronger wave in fall 2009. At its peak at the end of October, there were almost 250 daily hospital admissions and fewer than 20 daily deaths.

Source: Melissa Helferty et al., “Incidence of hospital admissions and severe outcomes during the first and second waves of pandemic (H1N1) 2009,” Canadian Medical Association Journal, December 14, 2010, volume 182, number 18, p. 1983.

Regional Second Waves versus a Global Second Wave

A second wave can emerge only once the first wave has subsided. Infection rates in countries across the globe are characterized by different patterns over different timelines.

According to a review of various countries on the World Health Organization COVID-19 dashboard, first waves appear to have ended with low and sustained steady state infection levels in some jurisdictions including Norway and Thailand.

Several countries are experiencing a second wave, such as many countries in Europe, Israel and Canada. Other countries, such as Australia, and Japan, have seen a second wave.

The United States experienced a resurgence of daily cases after a brief and slight decline in infection rates. This resurgence is not a second wave but rather a spike in infections within the first wave. As of the end of September 2020, some countries were still in the first wave, including Mexico, Brazil and India.

Globally, the daily infection rate has not yet shown a decline. While daily new infections plateaued at some 80,000 cases in April and May 2020, they have steadily risen since June, reaching 300,000 new infections per day. As a result, there is no clear first wave pattern yet from a global perspective.

What is the Historical Evidence for a Second Wave in a Pandemic?

Most historical evidence for pandemics spreading in waves comes from pandemics caused by influenza viruses. The COVID-19 pandemic is only the second one known to have been caused by a coronavirus.

The first one, the SARS pandemic of 2002-2003, had fewer infections and deaths than COVID-19, although the fatality rate was higher. Examples of these pandemics and epidemics are described in the table below.

Characteristics of Selected Viral Respiratory Pandemics and Epidemics, 1889 to 2020

Pandemic/Epidemic Responsible Virus Geographic Spread Infections/fatality Most Affected Population Pattern of spread
Influenza Viruses
Influenza pandemic of 1889-1892 Influenza type A virus, subtype H2 Global Infection rate unknown, one million deaths estimated Individuals with underlying health conditions Three waves, second most severe
Influenza pandemic of 1918-1920 Influenza type A virus, subtype H1N1 Global Estimated 500,000 million infected and 50-100 million deaths Most deaths were in people under 65 years, peak in young adults Three waves, second most severe
Influenza pandemic of 1957-1958 Influenza type A virus, subtype H2 Global Estimated 40% of the global population infected, 1.1 million deaths Children, the elderly and individuals with underlying health conditions Two waves, equally severe
Influenza pandemic of 1968-1969 Influenza type A virus, subtype H3 Global Estimated 15% infection rate, 1 million global deaths (0.5% mortality rate) Children, the elderly and individuals with underlying conditions Two waves
Influenza pandemic of 2009-2010 Influenza type A virus, subtype H1N1


Global 200 million infected, estimated 200,000 deaths (0.001% – 0.002% mortality rate) Adolescents and young adults Two mild waves, second slightly more severe
SARS epidemic of 2002-2003 (Severe Acute Respiratory Disease) SARS-CoV-11

(a coronavirus)

Southeast Asia and Canada About 8,500 infected, 800 deaths (mortality rate 9%) Elderly with underlying conditions. Two small waves
Coronavirus Respiratory Syndrome epidemic of 2012 (Middle East Respiratory Syndrome, MERS) * MERS-CoV


27 countries, mostly in the Arabian peninsula 2,494 infected, 858 deaths, (35% mortality rate) Men older than 60 years with underlying conditions. Multiple small waves
COVID-19 pandemic SARS-CoV-2 Global More than 33 million infected, more than 1 million deaths (average 4-5% case fatality rate, varies by country) Elderly and individuals with underlying conditions To be determined

*  Unlike the other viruses listed here, MERS-CoV is usually transmitted to humans from animals rather than from person-to-person contact.

Source: Prepared by author based upon Tom Jefferson and Carl Heneghan,Covid-19 – Epidemic ‘Waves’,” Centre for Evidence Based Medicine, 30 April 2020 and additional information in hyperlinked sources.

What Is The Likely Cause of a Second Wave of COVID-19?

There is currently no vaccine or treatment for COVID-19. The decrease in daily infections in Canada seen over the summer months was the result of the efforts of officials and individuals to reduce the capacity for the virus to spread. Efforts include travel restrictions and closed borders, restricted business activity, quarantine, physical distancing, use of masks, frequent handwashing and sanitizing, and staying home as much as possible.

According to government modelling released in July, relaxing population-based measures (e.g., business closures) without adequate personal public health action (e.g., physical distancing) and testing and tracing activity would likely result in a resurgence of cases.

Modifying public health restrictions during an epidemic or pandemic should ideally be done slowly and with appropriate health and safety protocols in place. Any increases in infection rates are then identified quickly and addressed through testing, contact tracing of positive cases and the isolation of individuals known or suspected to be infected.

However, if infection rates increase too much and contact tracing becomes difficult, containing infections becomes unlikely, resulting in increasing daily COVID-19 cases. Failure to contain new infections can result in a second wave of disease and would require the reimplementation of some of the restrictions.

Other issues could contribute to a potential second wave, including a viral mutation that increases the virus’ transmissibility or virulence as was the case in the 1918-1920 flu pandemic; lower than expected natural immunity to SARS-CoV-2 in previously infected individuals; and, increased susceptibility to SARS-CoV-2 infection or severe outcomes because of coinciding seasonal influenza, which itself will further burden the health care system.

The spring/fall waves previously seen in influenza pandemics are similar to the seasonal flu pattern. Children, who are major spreaders of the flu, are out of school during winter and summer holiday periods, reducing the spread of the virus. However, it is not clear whether children play a significant role in the spread of COVID-19.

Finally, spread of the novel coronavirus is known to be faster indoors than outdoors. As colder weather approaches, people will spend more time indoors, which may also lead to a resurgence in cases, especially if public health measures are not respected.

The Future of the COVID-19 Pandemic

Several global influenza pandemics have been characterized by waves of infection, but there is less evidence for coronavirus outbreaks showing such a pattern. Influenza pandemics such as seasonal influenza have generally occurred in the spring and autumn. Sometimes, the second wave of influenza pandemics has been more severe than the first.

The coronaviruses that caused SARS and MERS infected considerably fewer people than influenza viruses and did not show a defined two-wave infection pattern. However, the coronavirus causing COVID-19 has not behaved like the SARS and MERS coronaviruses. It has infected considerably more people and resulted in many more deaths, despite the disease’s lower fatality rate.

Many parts of the world are experiencing a second wave of COVID-19. It is likely that significant illness and death is yet to come as the world awaits effective treatments and vaccines.

Additional Resources

Alberto Aleta et al., “Modeling the impact of social distancing, testing, contact tracing and household quarantine on second-wave scenarios of the COVID-19 epidemic,” Center for Inference and Dynamics of Infectious Diseases, 18 May 2020 doi:

Inayat Ali, “COVID-19: Are We Ready for the Second Wave?,” Disaster Medicine and Public Health Preparedness,published online 7 May 2020, doi: 10.1017/dmp.2020.149. (Preprint)

Eskild Petersen et al., “Comparing SARS-CoV-2 with SARS-CoV and influenza pandemics,” The Lancet, published online 3 July 2020, doi.org/10.1016/S1473-3099(20)30484-9.

Shunqing Xu and Yuanyuan Li, “Beware of the second wave of COVID-19,” The Lancet, volume 395, issue 10233, 25 April-1 May 2020.

Worldometer, COVID-19 Coronavirus Pandemic.

Author: Sonya Norris, Library of Parliament

Categories: COVID-19, Health

Tags: , , , , ,

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