As countries across Europe start to lift their lockdowns, close attention is being paid to the so-called "R" rate as an indication of whether a second wave of coronavirus infections could be on the horizon.
Epidemiologists and governments are not just watching the daily number of cases recorded as restrictions are lifted, they are also keeping a close eye on this key metric in gauging the coronavirus' ability to spread.
Also known as the "basic reproductive number" or R value or rate, the figure indicates the number of secondary infections generated from one infected individual, on average, assuming there is no immunity among a population already.
Simply put, it gives experts an indication of the extent to which the coronavirus is being spread, or reproduced, among a population. The higher the reproduction rate, the higher the risk of a virus spreading exponentially.
The European Centre for Disease Prevention and Control (ECDC) told CNBC Thursday that it is important to keep the reproduction rate to less than 1, as this means that each person infected is likely to infect less than one person on average. "If this is the case, the outbreak can be contained," a spokesman for the center said.
Germany prompted concern Tuesday when it said that its R rate had increased up to 1.0, from 0.7 earlier in April, just as the country started to reopen its economy.
"The number should stay below one, that is the big goal," Lothar Wieler, the president of the Robert Koch Institute for infectious diseases (RKI) in Germany told a news conference Tuesday.
"The further it is below one, the more secure we can feel, the more leeway we have, but there are other numbers that are also important," he said, including the number of cases per day, and testing capacity. On Thursday, however, the RKI said the reproduction rate is currently estimated at 0.76 on average.
While Germany has seen a similar number of confirmed coronavirus cases to its European peers (161,539 cases Thursday, according to data compiled by Johns Hopkins University), it has seen far fewer deaths, with 6,467 fatalities recorded. By contrast, Italy, the U.K. and Spain have reported 27,682, 26,097 and 24,275 deaths respectively.
Apparently on top of its outbreak, but aware of the risks of lifting restrictions, Germany tentatively started to relax some lockdown measures last Monday by allowing smaller retail stores to re-open, with more restrictions set to be lifted over the coming months.
Asked whether the rise in the R rate should give Germany pause for thought when it comes to relaxing its lockdown, the Robert Koch Institute said the figure was just one of many factors to consider.
"The RKI does not decide on lockdown or exit," a spokesperson for the institute told CNBC Tuesday. "We provide the epidemiological basis, but the decision is on the political side — who have to consider and weigh all the other societal factors as well."
The spoksperson added that they had seen a decline in cases, adding: "I think all in all, we did comparatively well so far, and we were very lucky to see cases that early, but we still have to be very cautious and we have to observe closely how everything develops."
Estimating the reproduction value is not a precise science and different countries and experts use different statistical and mathematical models to estimate the extent to which the virus is being reproduced among a population.
Back in early March, for example, the World Health Organization said the reproductive number for Covid-19 was understood to be between 2 and 2.5, higher than for influenza, although it noted that direct comparisons are difficult.
The transmission of the coronavirus is not fully understood but social distancing and lockdown measures are aimed at restricting a virus' reproductive rate by limiting contact and potential infection between individuals.
Spain's leading epidemiologist and the country's health emergency chief, Fernando Simon, said Wednesday that unless the R rate was below 1 on average in Spain, and remained there, the country would not consider easing mobility restrictions. Almost all areas of Spain currently have a coronavirus reproduction rate below 1 currently, he noted.
The prime minister of Italy, which has been the hardest hit European country in terms of deaths from the virus, warned Tuesday that there was a "strong risk" of the coronavirus returning unless safety precautions were observed in the country's second phase of lifting lockdown measures, starting May 4.
The U.K., meanwhile, is still in full lockdown and is believed to be a week or so behind its European neighbors in terms of the outbreak's progression. Nonetheless, with 26,166 deaths from the virus as of Wednesday (when it first included deaths from Covid-19 in all settings and not just hospitals, leading to a sharp increase in the fatality figure), it is likely to be the worst hit country in Europe in terms of fatalities.
Under pressure to announce an exit strategy, it is watching its European neighbors carefully to see what happens as restrictions are lifted. On Wednesday, Foreign Minister Dominic Raab said "a second spike would be harmful to public health and it would result in many more deaths from Covid-19, that in itself would lead to a second lockdown inflicting further prolonged economic pain."
Boris Johnson is expected to tell the British public on Thursday that the R rate will be crucial yardstick of whether to lift, or reinstate, restrictions in the coming weeks, the BBC reported Thursday.