"lessen" interchangeable with "reduce"?

Short answer: Yes.

Longer answer:

According to the dictionary, the definitions for both are the following:

Reduce - make smaller or less in amount, degree, or size.

Lessen - make or become less, diminish.

You may notice that the word lessen is in fact a synonym of reduce and vice versa. It would be appropriate to say:

We must reduce the transmission rate of the virus.

or

We must lessen the transmission rate of the virus.

Here both work interchangeably. However, depending on the context, it may be more expressive to use one or the other. For example, when strictly referring to size, one might say "I would like to reduce the size of my garden," rather than saying "I would like to lessen the size of my garden" although technically correct.

Another example of interchangeability:

In order to make a more egalitarian society, we must reduce the wage gap.

Is the same as saying

In order to make a more egalitarian society, we must lessen the wage gap.

Lessen may also be more commonly used referring to qualitative (effects) rather than explicit quantifiable measures, although again they can essentially be used interchangeably.