Our results identified four trajectories of response in the spread of COVID-19 based on when the first response was initiated. Overall, governments’ responses were upgraded rapidly with further spread of COVID-19. However, these responses varied substantially across countries and regions. Shorter time interval to reach a high response level was associated with earlier arrival of a reduced peak of daily new case (as illustrated in Fig. 6).
An illustration of the relationship between timing of implementing a high response level and reduced peak of daily new cases. a When stringency index (SI) was low or was upgraded to a high level slowly; b When upgraded SI to a high level quickly
Different initiation stages and trajectory of SI reflects the gradual shifts in the spread of COVID-19 both in space and time [14], also reflects whether a government response timely and vigilantly. For example, social-control measures that worked for COVID-19 in areas such as Singapore, Japan and China came later in Europe and the USA.
The countries who started a response at the first stage initiated early, and kept a relatively low or moderate response level (usually with a SI < 60) for a long time, such as Japan and Singapore. The countries who started a response at the second stage have a wide difference in the date of initiation and have a mixed initiation pattern. For instance, China started a response and upgraded to a high response level in a short time interval (less than 2 weeks). Other countries upgraded to a high-level response in a stepwise fashion with mixed scale. Compared to the countries who initiated a response at the second stage, countries which initiated a response at the third stage had relatively shorter time span of initiation, and the interval between initiation and upgrading to a high level was also shorter than that of countries at the second stage. The countries at the fourth stage (after declaration of pandemic by WHO) started the response in a very short time span and had the shortest interval from initiation to a high response level.
Except a few countries from Asia which adopted measures earlier and maintained a moderate response throughout (e.g., Singapore and Japan), the majority of other countries who started a response at the second, third and fourth stage, upgraded their response to a high level only after WHO declared COVID-19 as a pandemic. This indicates announcement of COVID-19 pandemic has triggered countries to act more aggressively against COVID-19.
Four patterns of relationships between trajectory of SI and daily case number were identified in our analyses. First, some Asian countries and regions (e.g., Singapore, and Japan) started response early (at the first stage) and kept a low or moderate SI level for longer period. The early initiation of a response helped these countries to flatten the original peak of epidemic curve [15, 16]. Nevertheless, the low or moderate response level make the peak number of daily new cases (after measures adopted) come later. Although a low to moderate response level can fight against the virus’ spread without major disruption to daily living [15], it bears some risk. In some countries such as Singapore, even a small rebound of cases can seriously jeopardize the existing efforts in curbing disease transmission, and need a serious concerted public response [17].
Second, some countries initiated and upgraded the SI to a high level in a short interval, and kept it for a while, such as China, Denmark and the Netherlands. The reduced peak daily incidence in these countries came earlier than countries where the first pattern was found. This implies a quick and aggressive social control measures not only can flatten the peak of epidemic curve (as shown in previous literature) [18,19,20], it also make the turning point of the epidemic curve come earlier (as illustrated in Fig. 6). Third, unlike countries in the second pattern, some countries initiated like Italy and Russia have taken a long time before reaching a high response level. The reduced peak of daily incidence was relatively high and only achieved at a later stage. Fourth, some countries started a response only at a later stage, and maintained a relatively low SI level (without achieving a high response level), such as Sweden and USA. In these countries, even the peak daily incidence in epidemic curve under intervention was high (or stay on a plateau for a period) and came later.
Social-control measures, medications and a vaccine are key weapons against the pandemic [21, 22]. Before herd immunity was formed in population by largescale vaccination, social-control measures, combing classical epidemiological tactics such as isolating the sick, quarantining their contacts are still highly relevant in the fight against COVID-19 [23]. Studies have reported that non-pharmaceutical interventions such as social distancing have substantially flattened the original peak (when no social measure was adopted) and area under the epidemic curve [16, 20], thus reducing the pressure on the health system from high daily incidence of COVID-19 cases. This might have also reduced the burden on health care workers who are currently working beyond the point of exhaustion.
The interval from the first reported case to the time of high response level (SI > 80) reflects the agility of a government’s response—depicting how quickly a government adopt comprehensive and aggressive measures, following the recommendation by the WHO [24]. Upgrading the response level to a high level in a short time (usually with SI greater than 80), can help to achieve a reduced peak of daily new case sooner than later. This also indicates an earlier arrival of turning point in the epidemic curve that can potentially prevent collateral damage and support health system recovery.
Different from previous studies which have focused on the effectiveness of non-pharmaceutical interventions on preventing COVID-19, we quantified the relationship between the speed of reaching a high response level and the timing of the peak of epidemic curve. This implies that both timing and intensity of the governments’ response affect the pandemic. Sooner to upgrade to a high SI level might be the optimal option to curb COVID-19 pandemic.
There are several limitations for the data we used. First, although we have dealt missing date with imputation method, it might still be inadequate to capture “real” data. For example, in China, the sub-indicator of staying at home requirement was missing between February 3 and April 7, 2020. We imputed the degree of this indicator by searching the implement status of this indicator during this period in China. Also, being a big country with many provinces or states, one parameter in the calculation of SI for indicating whether a measure was adopted in the whole country level (general) or in a certain area level (targeted) might cause some bias. Provinces could differ in implementing or canceling the same measure based on their COVID-19 epidemic situation. Further, same requirement may be carried out differently at personal level based on acceptability of the measure in different countries. Last, the capacity of case detection, the intensity of PCR test to actively find people who were infected, and the accuracy of case report varied from country to country. These might have caused confounding bias to the parametric estimates of regression.
