Inferring the effectiveness of government interventions against COVID-19

1Worldwide, governments have mobilized resources to fight the COVID-19 pandemic. A wide range of nonpharmaceutical interventions (NPIs) has been deployed, including stay-at-home orders and the closure of all nonessential businesses. Recent analyses show that these large-scale NPIs were jointly effective at reducing the virus’s effective reproduction number Rt, but it is still largely unknown how effective individual NPIs were. As more data become available, we can move beyond estimating the combined effect of a bundle of NPIs and begin to understand the effects of individual interventions. This can help governments efficiently control the epidemic, by focusing on the most effective NPIs to ease the burden put on the population.

2A promising way to estimate NPI effectiveness is data-driven, cross-country modeling: inferring effectiveness by relating the NPIs implemented in different countries to the course of the epidemic in these countries. To disentangle the effects of individual NPIs, we need to leverage data from multiple countries with diverse sets of interventions in place. Previous data-driven studies (table S8) estimate effectiveness for individual countries or NPIs, although some exceptions do exist [; summarized in table S7]. In contrast, we evaluated the impact of several NPIs on the epidemic’s growth in 34 European and 7 non-European countries. If all countries implemented the same set of NPIs on the same day, the individual effect of each NPI would be unidentifiable. However, the COVID-19 response was far less coordinated: Countries implemented different sets of NPIs at different times and in different orders (Fig. 1).

3Even with diverse data from many countries, estimating NPI effects remains a challenging task. To begin with, models are based on uncertain epidemiological parameters; our NPI effectiveness study incorporates some of this uncertainty directly into the model. Furthermore, the data are retrospective and observational, meaning that unobserved factors could confound the results. Also, NPI effectiveness estimates can be highly sensitive to arbitrary modeling decisions, as shown by two recent replication studies. And finally, large-scale public NPI datasets suffer from frequent inconsistencies and missing data. Hence, the data and the model must be carefully validated if they are to be used to guide policy decisions. We have collected a large public dataset on NPI implementation dates that has been validated by independent double entry, and we have extensively validated our effectiveness estimates. This validation of data and model is a crucial but often absent or incomplete element of COVID-19 NPI effectiveness studies.

4Our results provide insight on the amount of COVID-19 transmission associated with various areas and activities of public life, such as gatherings of different sizes. Therefore, they may inform the packages of interventions that countries implement to control transmission in current and future waves of infections. However, we need to be careful when interpreting this study’s results. We only analyzed the effect NPIs had between January and the end of May 2020, and NPI effectiveness may change over time as circumstances change. Lifting an NPI does not imply that transmission will return to its original level, and our window of analysis does not include relaxation of NPIs. These and other limitations are detailed in the Discussion section.

5We analyzed the effects of seven commonly used NPIs between 22 January and 30 May 2020. All NPIs aimed to reduce the number of contacts within the population (Table 1). If a country lifted an NPI before 30 May, the window of analysis for that country terminates on the day of the lifting (see Materials and methods). To ensure high data quality, all NPI data were independently entered by two of the authors (independent double entry) using primary sources and then manually compared with several public datasets. Data on confirmed COVID-19 cases and deaths were taken from the Johns Hopkins Center for Systems Science and Engineering (CSSE) COVID-19 Dataset. The data used in this study, including sources, are available online.

6We estimated the effectiveness of NPIs with a Bayesian hierarchical model. We used case and death data from each country to infer the number of new infections at each point in time, which is itself used to infer the (instantaneous) reproduction number Rt over time. NPI effects were then estimated by relating the daily reproduction numbers to the active NPIs, across all days and countries. This relatively simple, data-driven approach allowed us to sidestep assumptions about contact patterns and intensity, infectiousness of different age groups, and so forth that are typically required in modeling studies. This approach also allowed us to directly model many sources of uncertainty, such as uncertain epidemiological parameters, differences in NPI effectiveness between countries, unknown changes in testing and infection fatality rates, and the effect of unobserved influences on Rt. The code is available online.

7Our model enabled us to estimate the individual effectiveness of each NPI, expressed as a percentage reduction in Rt. We quantified uncertainty with Bayesian prediction intervals, which are wider than standard credible intervals. Bayesian prediction intervals reflect differences in NPI effectiveness across countries among several other sources of uncertainty. They are analogous to the standard deviation of the effectiveness across countries rather than the standard error of the mean effectiveness. Under the default model settings, the percentage reduction in Rt (with 95% prediction interval; Fig. 2) associated with each NPI was as follows: limiting gatherings to 1000 people or less: 23% (0 to 40%); limiting gatherings to 100 people or less: 34% (12 to 52%); limiting gatherings to 10 people or less: 42% (17 to 60%); closing some high-risk face-to-face businesses: 18% (−8 to 40%); closing most nonessential face-to-face businesses: 27% (−3 to 49%); closing both schools and universities in conjunction: 38% (16 to 54%); and issuing stay-at-home orders (additional effect on top of all other NPIs): 13% (−5 to 31%). Note that we were not able to robustly disentangle the individual effects of closing only schools or only universities, because these NPIs were implemented on the same day or in close succession in most countries [except Iceland and Sweden, where only universities were closed (see also fig. S21)]. We thus reported “schools and universities closed” as one NPI.

8Some NPIs frequently co-occurred, i.e., were partly collinear. However, we were able to isolate the effects of individual NPIs, because the collinearity was imperfect and our dataset large. For every pair of NPIs, we observed one without the other for 504 days across all countries (country-days) on average (table S5). The minimum number of country-days for any NPI pair is 148 (for limiting gatherings to 1000 or 100 attendees). Additionally, under excessive collinearity, and insufficient data to overcome it, individual effectiveness estimates would be highly sensitive to variations in the data and model parameters. Indeed, high sensitivity prevented Flaxman et al., who had a smaller dataset, from disentangling NPI effects. In contrast, our effectiveness estimates are substantially less sensitive (see below). Finally, the posterior correlations between the effectiveness estimates are weak, further suggesting manageable collinearity (fig. S22).

9Although the correlations between the individual estimates were weak, we took them into account when evaluating combined NPI effectiveness. For example, if two NPIs frequently co-occur, there may be more certainty about the combined effectiveness than about the effectiveness of each NPI individually. Figure 3 shows the combined effectiveness of the sets of NPIs that are most common in our data. In combination, the NPIs in this study reduced Rt by 77% (67 to 85%). Across countries, the mean Rt without any NPIs (i.e., the R0) was 3.3 (table S4). Starting from this number, the estimated Rt likely could have been brought below 1 by closing schools and universities, closing high-risk businesses, and limiting gathering sizes to at most 10 people. Readers can interactively explore the effects of sets of NPIs with our online mitigation calculator. A comma-separated value file containing the joint effectiveness of all NPI combinations is available online.

10We performed a range of validation and sensitivity experiments (figs. S2 to S19). First, we analyzed how the model extrapolated to countries that did not contribute data for fitting the model, and we found that it could generate calibrated forecasts for up to 2 months, with uncertainty increasing over time. Multiple sensitivity analyses showed how the results changed when we modified the priors over epidemiological parameters, excluded countries from the dataset, used only deaths or confirmed cases as observations, varied the data preprocessing, and more. Finally, we tested our key assumptions by showing results for several alternative models [structural sensitivity] and examined possible confounding of our estimates by unobserved factors influencing Rt. In total, we considered NPI effectiveness under 206 alternative experimental conditions (Fig. 4A). Compared with the results obtained under our default settings (Figs. 2 and 3), median NPI effectiveness varied under alternative plausible experimental conditions. However, the trends in the results are robust, and some NPIs outperformed others under all tested conditions. Although we tested large ranges of plausible values, our experiments did not include every possible source of uncertainty.

11We categorized NPI effects into small, moderate, and large, which we define as a posterior median reduction in Rt of <17.5%, between 17.5 and 35%, and >35%, respectively (vertical lines in Fig. 4). Four of the NPIs fell into the same category across a large fraction of experimental conditions: closing both schools and universities was associated with a large effect in 96% of experimental conditions, and limiting gatherings to 10 people or less had a large effect in 99% of conditions. Closing most nonessential businesses had a moderate effect in 98% of conditions. Issuing stay-at-home orders (that is, in addition to the other NPIs) fell into the “small effect” category in 96% of experimental conditions. Three NPIs fell less clearly into one category: Limiting gatherings to 1000 people or less had a small-to-moderate effect (moderate in 81% of conditions) while limiting gatherings to 100 people or less had a moderate-to-large effect (moderate in 66% of conditions). Finally, closing some high-risk businesses, including bars, restaurants, and nightclubs, had a small-to-moderate effect (moderate in 58% of conditions). Limiting gatherings to 1000 people or less was the NPI with the highest variation in median effectiveness across the experimental conditions (Fig. 4A), which may reflect this NPI’s partial collinearity with limiting gatherings to 100 people or less.

12Aggregating all sensitivity analyses can hide sensitivity to specific assumptions. We display the median NPI effects in four categories of sensitivity analyses (Fig. 4, B to E), and each individual sensitivity analysis is shown in the supplementary materials. The trends in the results are also stable within these categories.

13We used a data-driven approach to estimate the effects that seven nonpharmaceutical interventions had on COVID-19 transmission in 41 countries between January and the end of May 2020. We found that several NPIs were associated with a clear reduction in Rt, in line with mounting evidence that NPIs are effective at mitigating and suppressing outbreaks of COVID-19. Furthermore, our results indicate that some NPIs outperformed others. While the exact effectiveness estimates vary with modeling assumptions, the broad conclusions discussed below are largely robust across 206 experimental conditions in 11 sensitivity analyses.

14Business closures and gathering bans both seem to have been effective at reducing COVID-19 transmission. Closing most nonessential face-to-face businesses was only somewhat more effective than targeted closures, which only affected businesses with high infection risk, such as bars, restaurants, and nightclubs (see also Table 1). Therefore, targeted business closures can be a promising policy option in some circumstances. Limiting gatherings to 10 people or less was more effective than limits of up to 100 or 1000 people and had a more robust effect estimate. Note that our estimates are derived from data between January and May 2020, a period when most gatherings were likely indoors owing to the weather.

15Whenever countries in our dataset introduced stay-at-home orders, they essentially always also implemented, or already had in place, all other NPIs in this study. We accounted for these other NPIs separately and isolated the effect of ordering the population to stay at home, in addition to the effect of all other NPIs. In accordance with other studies that took this approach, we found that issuing a stay-at-home order had a small effect when a country had already closed educational institutions and nonessential businesses and had banned gatherings. In contrast, Flaxman et al. and Hsiang et al. included the effect of several NPIs in the effectiveness of their stay-at-home order (or “lockdown”) NPIs and accordingly found a large effect for this NPI. Our finding suggests that some countries may have been able to reduce Rt to <1 without a stay-at-home order (Fig. 3) by issuing other NPIs.

16We found a large effect for closing both schools and universities in conjunction, which was remarkably robust across different model structures, variations in the data, and epidemiological assumptions (Fig. 4). This effect remained robust when controlling for NPIs excluded from our study (fig. S9). Our approach cannot distinguish direct effects on transmission in schools and universities from indirect effects, such as the general population behaving more cautiously after school closures signaled the gravity of the pandemic. Additionally, because school and university closures were implemented on the same day or in close succession in most of the countries we studied, our approach cannot distinguish their individual effects (fig. S21). This limitation likely also holds for other observational studies that do not include data on university closures and estimate only the effect of school closures. Furthermore, our study does not provide evidence on the effect of closing preschools and nurseries.

17Previous evidence on the role of pupils and students in transmission is mixed. Although infected young people (~12 to 25 years of age) are often asymptomatic, they appear to shed similar amounts of virus as older people and might therefore infect higher-risk individuals. Early data suggested that children and young adults had a notably lower observed incidence rate than older adults—whether this was due to school and university closures remains unknown. In contrast, the recent resurgence of cases in European countries has been concentrated in the age group corresponding to secondary school and higher education (especially the latter) and is now spreading to older age groups as well as primary school–aged children. Primary schools may be generally less affected than secondary schools, perhaps partly because children under the age of 12 are less susceptible to SARS-CoV-2.

18Our study has several limitations. (i) NPI effectiveness may depend on the context of implementation, such as the presence of other NPIs, country demographics, and specific implementation details. Our results thus need to be interpreted as indicating the effectiveness in the contexts in which the NPI was implemented in our data. For example, in a country with a comparatively old population, the effectiveness of closing schools and universities would likely have been on the lower end of our prediction interval. Expert judgment should thus be used to adjust our estimates to local circumstances. (ii) Rt may have been reduced by unobserved NPIs or voluntary behavior changes such as mask-wearing. To investigate whether the effect of these potential confounders could be falsely attributed to the observed NPIs, we performed several additional analyses and found that our results are stable to a range of unobserved factors (fig. S9). However, this sensitivity check cannot provide certainty, and investigating the role of unobserved factors is an important topic to explore further. (iii) Our results cannot be used without qualification to predict the effect of lifting NPIs. For example, closing schools and universities in conjunction seems to have greatly reduced transmission, but this does not mean that reopening them will necessarily cause infections to soar. Educational institutions can implement safety measures, such as reduced class sizes, as they reopen. However, the nearly 40,000 confirmed cases associated with universities in the United Kingdom since they reopened in September 2020 show that educational institutions may still play a large role in transmission, despite safety measures. (iv) We do not have data on some promising interventions, such as testing, tracing, and case isolation. These interventions could become an important part of a cost-effective epidemic response, but we did not include them because it is difficult to obtain comprehensive data on their implementation. In addition, although the data are more readily available, it is difficult to estimate the effect of mask-wearing in public spaces because there was limited public life as a result of other NPIs. We discuss further limitations in supplementary text section E.

19Although our work focused on estimating the impact of NPIs on the reproduction number Rt, the ultimate goal of governments may be to reduce the incidence, prevalence, and excess mortality of COVID-19. For this, controlling Rt is essential, but the contribution of NPIs toward these goals may also be mediated by other factors, such as their duration and timing, periodicity and adherence, and successful containment. While each of these factors addresses transmission within individual countries, it can be crucial to also synchronize NPIs between countries, given that cases can be imported.

20Many governments around the world seek to keep Rt below 1 while minimizing the social and economic costs of their interventions. Our work offers insights into which areas of public life are most in need of virus containment measures so that activities can continue as the pandemic develops; however, our estimates should not be taken as the final word on NPI effectiveness.

21We analyzed the effects of NPIs (Table 1) in 41 countries (Fig. 1). We recorded NPI implementations when the measures were implemented nationally or in most regions of a country (affecting at least three-fourths of the population). We recorded only mandatory restrictions, not recommendations. Supplementary text section G details how edge cases in the data collection were handled. For each country, the window of analysis starts on 22 January and ends either after the first lifting of an NPI or on 30 May 2020, whichever came first. The reason to end the analysis after the first major reopening was to avoid a distribution shift. For example, when schools reopened, it was often with safety measures, such as smaller class sizes and distancing rules. It is therefore expected that contact patterns in schools will have been different before school closure compared with after reopening. Modeling this difference explicitly is left for future work. Data on confirmed COVID-19 cases and deaths were taken from the Johns Hopkins CSSE COVID-19 Dataset. The data used in this study, including sources, are available online.

22We collected data on the start and end dates of NPI implementations, from the start of the pandemic until 30 May 2020. Before collecting the data, we experimented with several public NPI datasets, finding that they were not complete enough for our modeling and contained incorrect dates. By focusing on a smaller set of countries and NPIs than these datasets, we were able to enforce strong quality controls: We used independent double entry and manually compared our data with public datasets for cross-checking.

23First, two authors independently researched each country and entered the NPI data into separate spreadsheets. The researchers manually researched the dates using internet searches: There was no automatic component in the data-gathering process. The average time spent researching each country was 1.5 hours per researcher. Next, the researchers independently compared their entries against two public datasets, the Epidemic Forecasting Global NPI (EFGNPI) Database and the Oxford COVID-19 Government Response Tracker, and, if there were conflicts, visited all primary sources to resolve the conflicts. After that, each country and NPI was again independently entered by one to three paid contractors, who were provided with a detailed description of the NPIs and asked to include primary sources with their data. A researcher then resolved any conflicts between this data and one (but not both) of the spreadsheets. Finally, the two independent spreadsheets were combined and all conflicts resolved by a researcher. The final dataset contains primary sources (government websites and/or media articles) for each entry.

24When the case count is small, a large fraction of cases may be imported from other countries and the testing regime may change rapidly. To prevent this from biasing our model, we neglected case numbers before a country had reached 100 confirmed cases and fatality numbers before a country had reached 10 deaths. We included these thresholds in our sensitivity analysis (fig. S13).

25In this section, we give a short summary of the model (Fig. 5). The detailed model description is given in supplementary text section A. Briefly, our model uses case and death data from each country to “backward” infer the number of new infections at each point in time, which is itself used to infer the reproduction numbers. NPI effects are then estimated by relating the daily reproduction numbers to the active NPIs, across all days and countries. This relatively simple, data-driven approach allowed us to sidestep assumptions about contact patterns and intensity, infectiousness of different age groups, and so forth that are typically required in modeling studies. Code is available online.

26Our model builds on the semimechanistic Bayesian hierarchical model of Flaxman et al., with several additions. First, we allow our model to observe both case and death data. This increases the amount of data from which we can extract NPI effects, reduces distinct biases in case and death reporting, and reduces the bias from including only countries with many deaths. Second, since epidemiological parameters are only known with uncertainty, we place priors over them, following recent recommended practice. Third, as we do not aim to infer the total number of COVID-19 infections, we can avoid assuming a specific infection fatality rate (IFR) or ascertainment rate (rate of testing). Fourth, we allow the effects of all NPIs to vary across countries, reflecting differences in NPI implementation and adherence.

27We now describe the model by going through Fig. 5 from bottom to top. The growth of the epidemic is determined by the time- and country-specific reproduction number Rt,c, which depends on (i) the (unobserved) basic reproduction number in country c, R0,c, and (ii) the active NPIs at time t. R0,c accounts for all time-invariant factors that affect transmission in country c, such as differences in demographics, population density, culture, and health systems.

28Following Flaxman et al. and others, each NPI is assumed to independently affect Rt,c as a multiplicative factorwhere xi,t,c = 1 indicates that NPI i is active in country c on day t (xi,t,c = 0 otherwise), I is the number of NPIs, and αi,c is the effect parameter for NPI i in country c. The multiplicative effect encodes the plausible assumption that NPIs have a smaller absolute effect when Rt,c is already low.

29We assume that the effect of each NPI on Rt,c is stable across time but can vary across countries to some degree. Concretely, the effect parameter of intervention i in country c is defined as αi,c = αi + zi,c, where αi represents the mean effect parameter, and. The variance σi2 corresponds to the degree of cross-country variation in the effectiveness of NPI i and is inferred from the data. This partial pooling of NPI effect parameters minimizes bias from country-specific sources while also reflecting that NPI effectiveness is likely different across countries. We define the effectiveness of NPI i as the percentage reduction in Rt associated with NPI i across countries. This effectiveness, displayed in Figs. 2 to 4, is computed as 1 – exp(–(αi + zi)), where again and σi2 is drawn from its posterior. We place an asymmetric Laplace prior on αi that allows for both positive and negative effects but places 80% of its probability mass on positive effects, reflecting that NPIs are more likely to reduce Rt,c than to increase it.

30In the early phase of an epidemic, the number of new daily infections grows exponentially. During exponential growth, there is a one-to-one correspondence between the daily growth rate and Rt,c. The correspondence depends on the generation interval (the time between successive infections in a chain of transmission), which we assume to have a gamma distribution. The prior on the mean generation interval has a mean of 5.06 days, derived from a meta-analysis.

31We model the daily new infection count separately for confirmed cases and deaths, representing those infections that are subsequently reported and those that are subsequently fatal. However, both infection numbers are assumed to grow at the same daily rate in expectation, allowing the use of both data sources to estimate each αi. The infection numbers translate into reported confirmed cases and deaths after a delay. The delay is the sum of two independent distributions, assumed to be equal across countries: the incubation period and the delay from onset of symptoms to confirmation. We put priors over the means of both distributions, resulting in a prior over the mean infection-to-confirmation delay with a mean of 10.92 days (see supplementary text section A.3). Similarly, the infection-to-death delay is the sum of the incubation period and the delay from onset of symptoms to death, and the prior over its mean has a mean of 21.8 days. Finally, as in related models, both the reported cases and deaths follow a negative binomial output distribution with separate inferred dispersion parameters for cases and deaths.

32Using a Markov chain Monte Carlo (MCMC) sampling algorithm, this model infers posterior distributions of each NPI’s effectiveness while accounting for cross-country variations in effectiveness, reporting, and fatality rates as well as uncertainty in the generation interval and delay distributions. To analyze the extent to which modeling assumptions affect the results, our sensitivity analysis included all epidemiological parameters, prior distributions, and many of the structural assumptions introduced above. MCMC convergence statistics are shown in fig. S19.