France has entered the fourth wave of the Covid-19 epidemic largely due to the more contagious Delta variant of Sars-Cov-2. While fully vaccinated individuals are well protected against the new variant, experts feel the fact that a large number of people are still not vaccinated, or partially vaccinated, means vaccination alone won't be sufficient to break this wave.
“It is extremely important to get hold of the epidemic and control it. In the short term, it means breaking the transmission chain through measures such as testing, contact tracing, isolation, avoiding gatherings in closed spaces,” Dr Samuel Alizon, a CNRS research director specialising in the modelling of infectious disease dynamics, told RFI.
Dr Alizon pointed out that in the short term the exponential growth of cases due to the Delta variant cannot be matched by the linear progression of vaccination. He also said that in the medium and long term, large scale vaccination is the key to halt the spread of the disease.
What constitutes a wave?
The state of an epidemic is determined by the so-called Reproduction Number (R) which is the average number of people an infected person passes on the virus to.
According to Dr Alizon, if the R number is greater than 1 the epidemic grows and when it remains as such for a certain amount of time, it constitutes what is described as a wave. The current R number -- at 1.5 as per the tests data and 1.2-1.3 as per the hospital admissions -- stands higher than that of the third wave where it didn't cross 1.2.
Given that the Delta variant is more contagious than the previous strains, the high rise in the R number isn't surprising. “It appears the Delta variant is spreading fast. More precisely, our team has shown that it is spreading approximately 70 percent faster than the alpha variant.”
Just to get an idea of how contagious the Delta variant is, in the absence of immunity or controls (vaccination, social distancing measures, etc), an infected person would pass on the virus to between six and nine people. For the original strain, this number was three, which indicates that the new variant is between two to three times more contagious.
Vaccine protection against the Delta variant
Despite the large-scale vaccination drive in France, a significant percent of the population isn't yet fully vaccinated , making them vulnerable to the Delta variant.
“Forty percent of the population in France is still not vaccinated. Based on the R, we estimate that one-third of the non-vaccinated people might get infected by this fast spreading variant. Moreover, we are still unsure whether the new variant is able to evade natural immunity which would mean that those who had been infected earlier and are not fully vaccinated, could catch the virus again.”
Dr Alizon added that even those who have received one of the two doses of the vaccine are only partially protected. However, the recipients of double vaccine doses have more than 95 percent protection against severe Covid-19 as well as 60 to 80 percent protection against reinfection.
If the epidemic continues to spread at the same rate, hospitals in France will start feeling the strain by the middle of next month. "If the Reproduction Number remains the same, the hospitals will have a problematic situation by the end of August”, Dr Alizon contends.
He insists that measures to break the transmission chain should be implemented as early as possible. "The earlier the government decides to implement measures, the more efficient they are. They also disrupt our lives much less. But the longer you wait, the less options you have. The measures impact our daily lives more and last longer.”