The delta mutation is also spreading in Germany. In late summer, a fourth wave of transmissions is to be expected, as this variant is even more contagious than the currently prevailing alpha variant. We should be prepared for this possibility and be able to react quickly. Rapid tests should continue to be carried out in schools after the summer vacations in order to have a systematic overview of the incidence of infection (cf. Section 3, p. 2).

Evaluations of county-level mobility data on the effect of curfew restrictions during the federal "emergency brake" show that nighttime activity was reduced in North Rhine-Westphalia and some other regions of southwestern Germany. In terms of total time spent away from home, a reduction can be seen on weekends (cf. Section 4, p. 4).

With our mathematical model, we show how strong entry control or quarantine measures have to be designed to delay the introduction and spread in Germany of a corona mutation with the characteristics of the delta variant. To delay the spread by 20 days, the probability of introduction must be reduced by a factor of 10 (see Section 5, p. 7).

Our calculations show that a full opening of schools after the summer vacations without any protective measures would result in an exponential increase in transmission among students, assuming that they are not vaccinated by then (cf. Section 6.1, p. 11).

Since vaccinations provide better protection than a combination of rapid tests and masks, they can in principle replace the mask requirement. However, an urgent recommendation to wear FFP2 masks for unvaccinated persons indoors for self-protection is still appropriate (cf. Section 6.2, p. 12).
