Summary: Infectious disease specialists estimate that between 40% and 70% of adults could become infected with the new coronavirus during the course of this outbreak. In a recent interview, Dr. Marc Lipsitch, director of the Center for Communicable Disease Dynamics at Harvard T.H. Chan School of Public Health, explains the risks and transmission potential of COVID-19 and discusses what is known so far about how the virus may affect children.
Source: Harvard
As COVID-19 cases spread globally, U.S. officials acknowledged this week that coronavirus infections are likely to become far more widespread across the country. That development comes amid rapid changes: reports of vaccine candidates, a shift in the majority of new cases to countries outside China, emergence of infections in locations such as California and Germany with no clear source, extended school closures in Japan, and volatility in global financial markets driven by economic concerns. Public health experts have also noted encouraging signs that China’s extreme control measures—strict travel limits, city lockdowns, and closure of many workplaces and schools—appear to have reduced transmission in parts of the country.
Harvard spoke with Marc Lipsitch, an epidemiologist who leads the Center for Communicable Disease Dynamics at Harvard T.H. Chan School of Public Health, about how the epidemic might evolve and what remains uncertain, particularly regarding children and transmission.
Harvard: For the first time, more new cases were reported outside of China than inside. Is this a meaningful turning point or just daily fluctuation?
LIPSITCH: It’s hard to be certain from one day’s numbers. I would prefer to see the trend continue for several days before calling it an inflection point. That said, the evidence increasingly supports the conclusion that China’s intense social distancing measures have significantly reduced transmission in many areas. The WHO mission’s findings support this, and my own information points the same way. That is encouraging, but it doesn’t mean China or any other country is free from future resurgences. Few places can maintain such strict distancing indefinitely, and as restrictions ease there is a risk of renewed spread.
Harvard: What surprised you most in recent developments?
LIPSITCH: I was struck by the sudden emergence of sizable clusters in places that weren’t obvious candidates for large outbreaks, such as Iran and Italy. In contrast, I had expected to see larger, easily detectable outbreaks in places like the United States. One part of the explanation may be limited testing: if testing is not widespread, many cases remain undetected. Even so, the outbreaks seen in Iran and Italy occurred with only moderate levels of testing, so it is somewhat surprising we haven’t yet seen an unmistakable outbreak in the U.S.
Harvard: Should routine testing be implemented here?
LIPSITCH: I recommend starting some routine testing. It doesn’t make sense to test everyone at large scale before there is clear evidence of community transmission, but we need to sample more broadly than only known contacts. Other regions are testing widely: Hong Kong tests every hospitalized patient with a cough and every unexplained pneumonia case, and Guangdong reportedly tested hundreds of thousands of people with mild respiratory symptoms over several weeks. Those are the scales at which a serious surveillance effort operates. We don’t have to match that scale everywhere immediately, but without broader testing we won’t know whether undetected community transmission is occurring.
Harvard: When does an epidemic become a pandemic?
LIPSITCH: The labels can be less helpful than focusing on the reality. A pandemic means sustained transmission in many places around the world. With sustained outbreaks in countries like Iran, Italy, Japan, South Korea, and China, we meet that practical definition. The debate over the specific word matters less than preparing for and responding to widespread transmission.
Harvard: You’ve previously said you expect 40% to 70% of humanity could be infected within a year. Is that still your estimate?
LIPSITCH: My estimate remains that 40% to 70% of adults could become infected over a year if effective long-term countermeasures are not implemented. An important caveat is that this range refers to adults; we do not yet understand whether children are being infected at lower rates or are simply experiencing milder or asymptomatic infections. The projection also assumes that widespread, sustained social distancing measures will not be maintained for extended periods in most places.
Harvard: What do we know about which demographic groups are most affected?
LIPSITCH: Older adults have higher risk of symptomatic and severe illness, including death, and men appear to be overrepresented among severe cases. Health-care workers face higher exposure, and we need more data on whether high exposure leads to more severe outcomes in that group. Anecdotes of younger clinicians becoming seriously ill raise questions about dose of exposure and severity, which require systematic investigation.
Harvard: Moderna recently delivered a vaccine candidate to the NIH for human testing. Could that shorten the typical one-year minimum timeline to develop and distribute a vaccine?
LIPSITCH: Rapid vaccine development is promising, but regulatory standards and careful trials exist for a reason. It might be possible to accelerate deployment more than usual, but rushing an unproven vaccine risks harm as well as ineffectiveness. We should move as quickly as possible while maintaining rigorous evaluation to ensure a vaccine is safe and beneficial before widespread use.
Harvard: The CDC said an outbreak in the U.S. is very likely and suggested social distancing as a tactic. Can social distancing make a meaningful difference without a treatment or vaccine?
LIPSITCH: Yes. Social distancing can slow transmission and is a viable tool for reducing peak demand on health care systems, lowering total infections, and buying time to learn more about treatments and preventive measures. Historical examples, such as responses during the 1918 influenza pandemic, show that social distancing can work. The key questions are how much distancing can be sustained and for how long. Even temporary measures that delay infections can have important public health benefits.
“We just don’t understand whether children are getting infected at low rates or just not showing very strong symptoms. So I don’t want to make assumptions about children until we know more.”
Harvard: What did you make of the administration’s remarks that the U.S. is prepared to meet this challenge?
LIPSITCH: Initially I left the briefing cautiously optimistic because officials emphasized public health leadership and outlined priorities such as expanding local response capacity. However, actions that limit public health experts from speaking openly are concerning. Public health decisions should be driven by science and transparent communication, and silencing federal health and science officials undermines public trust and effective response.
Harvard: With the Tokyo Olympics scheduled for July, is it possible now to judge whether a major international event should go ahead?
LIPSITCH: It’s too early to make a definitive call. The coming weeks will reveal how widespread global transmission becomes. If the virus is circulating everywhere, travel restrictions make less sense, but large gatherings could still pose high risk and may need to be reconsidered.
Harvard: What is the most important unanswered question right now?
LIPSITCH: One of the biggest uncertainties is the role children play in transmission. School closures are a common response in influenza pandemics, but for COVID-19 we need evidence on whether closing schools would reduce spread. School closures are costly and disruptive, especially where students rely on school meals and caretaking structures. We need careful household and community studies to determine whether children exposed to infected people become infected themselves, whether they shed virus, and whether that virus is infectious to others. Another priority is understanding the degree of undetected community spread in places where testing is limited.
Harvard: What do we know for certain about children and this virus?
LIPSITCH: Confirmed cases among children who are sick enough to be tested are much lower per capita than among adults. In regions of China outside Hubei province, children made up a larger share of reported cases than in Hubei, which suggests that when health systems are less overwhelmed, milder pediatric cases are more likely to be identified. This pattern implies either that children are infected but less symptomatic, or that children are less susceptible to infection—an important distinction that requires further study.
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